
Abstract
Conway's field of surreal numbers comes both with a natural total order and an additional “simplicity relation” which is also a partial order. Considering as a doubly ordered structure for these two orderings, an isomorphic copy of into itself is called a surreal substructure. It turns out that many natural subclasses of are actually of this type. In this paper, we study various constructions that give rise to surreal substructures and analyze important examples in greater detail.
The class of surreal numbers was discovered by Conway and studied in his wellknown monograph On Numbers and Games [11]. Conway's original definition is somewhat informal and goes at follows:
“If and are any two sets of (surreal) numbers, and no member of is any member of , then there is a (surreal) number . All (surreal) numbers are constructed in this way.”
The magic of surreal numbers lies in the fact that many traditional operations on integers and real numbers can be defined in a very simple way on surreal numbers. Yet, the class turns out to admit a surprisingly rich algebraic structure under these operations. For instance, the sum of two surreal numbers and is defined recursively by
In section 3 below, we recall similar definitions for subtraction and multiplication. Despite the fact that the basic arithmetic operations can be defined in such an “effortless” way, Conway showed that actually forms a realclosed field that contains . Strictly speaking, some care is required here, since the surreal numbers form a proper class. In particular, it contains all ordinal numbers . We refer to appendix B for ways to deal with this kind of settheoretic issues.
One convenient way to rigourously introduce surreal numbers is to regard them as “sign sequences” indexed by the elements of an ordinal number , called the length of : see section 2.1 below for details. Every ordinal itself is represented as with for all . The number is represented by the sign sequence of length . The ordering on corresponds to the lexicographical ordering on sign sequences, modulo zero padding when comparing two surreal numbers of different lengths. The sign sequence representation also induces the important notion of simplicity: given , we say that is simpler as , and write , if the sign sequence of is a truncation of the sign sequence of . The simplicity relation is denoted by in some previous works [8, 27, 3].
The sign sequence representation was introduced and studied systematically in Gonshor's book [21]. As we will see in section 3, it also allows for a natural extension of ordinal arithmetic to the surreal numbers. In order to avoid confusion, we will systematically use the notations and for ordinal sums and products and for ordinal exponentiation. For instance, in , we have . Given an ordinal , it is also natural to define the set of all surreal numbers of length . It turns out that is a realclosed subfield of if and only if is an number, i.e. [12, Proposition 4.7 and Corollary 4.9].
Quite some work has been dedicated to the extension of basic calculus to the surreal numbers and to the study of various operations in terms of sign sequences. In his book [21], Gonshor shows how to extend the real exponential function to . This exponential function actually admits the same first order properties as the usual exponential function: the class is elementarily equivalent to as an exponential field. In fact, they are even elementarily equivalent as real exponential ordered fields equipped with restricted analytic functions [12, Theorem 2.1]. Here we recall that a restricted real analytic function is a power series at the origin that converges on a small closed ball with . Then it can be shown that the definition of extends to surreal numbers with .
Another important question concerns the possibility to define a natural derivation on the surreal numbers, which is nontrivial in the sense that . Such a derivation was first constructed by Berarducci and Mantova [8], while making use of earlier work by van der Hoeven and his student Schmeling [35]. It was shown in [3] that this “Italian” derivation has “similarly good properties” as the exponential function in the sense that is elementary equivalent to the field of transseries as an Hfield. Here transseries are a generalization of formal power series. They form an ordered exponential field that comes with a derivation. The notion of an Hfield captures the algebraic properties of this field as well as those of socalled Hardy fields. We refer to [1] for more details.
The above results on the exponential function and the Italian derivation on rely on yet another representation of surreal numbers as generalized power series with real coefficients and monomials such that is simpler than any other with the same valuation as : see section 2.3 for details. Indeed, ordinary power series and Laurent series in can be regarded as functions in , so they come with a natural derivation. More generally, the exponential function on makes it possible to interpret any transseries in as a surreal number, which makes it again possible to derive such surreal numbers in a natural way.
Unfortunately, not all surreal numbers are transseries in . For instance, the surreal number is larger than any transseries in . In order to be able to intepret all surreal numbers as functions in , two ingredients are missing: on the one hand, we need to introduce ordinal “iterators” of the exponential function that grow faster than finite iterates. For instance, we have . On the other hand, we need to be able to represent socalled nested transseries such as
(1.2) 
The present paper is part of an ongoing project to represent any surreal number as a generalized “hyperseries” in , which takes these observations into account. This project was first mentioned in [26] and further detailed in [2]. For progress on the “series side”, we refer to [23, 35, 26, 13]. The derivation cannot be compatible with a composition law on [9, Theorem 8.4]. More specifically, it was noted in [2] that the Italian derivation fails to satisfy for all . Ultimately, the ability to represent surreal numbers as hyperseries evaluated at should lead to compatible definitions of a derivation and a composition on .
In the course of the above project to construct an isomorphism between and a suitable class of hyperseries, one frequently encounters subclasses of that are naturally parameterized by itself. For instance, Conway's generalized ordinal exponentiation is bijective, which leads to a natural parameterization of the class of monomials by (see Theorems 5.2 and 5.11). Similarly, nested expressions such as (1.2) do not give rise to a single surreal number, but rather to a class of surreal numbers that is naturally parameterized by (see Theorem 8.8). Yet another example is the class of logatomic surreal numbers that occurs crucially in the construction of derivations on [8, Section 5.2].
In these three examples, the parameterizations turn out to be more than mere bijective maps: they actually preserve both the ordering and the simplicity relation . This leads to the definition of a surreal substructure of as being an isomorphic copy of inside itself. Surreal substructures such as , , and behave similarly as the surreal numbers themselves in many regards. In our project, we have started to exploit this property for the definition and study of new functions on such as hyperlogarithms and nested transseries.
The main goal of the present paper is to develop the basic theory of surreal substructures for its own sake and as a new tool to study surreal numbers. We hope to convey the sense that surreal substructures are at the same time very general and very rigid subclasses of and that several problems regarding the enriched structure of (highlighted in particular in the work of Gonshor [21], Lemire [28, 29, 30], Ehrlich [16, 15, 17], Kuhlmann–Matusinski [27], Berarducci–Mantova [8], and Aschenbrenner–van den Dries–van der Hoeven [3]) crucially involve surreal substructures. Even for very basic subclasses of , such as , we suggest that it deserves our attention when they form surreal substructures.
A substantial part of our paper (namely, sections 4, 5, and 6) is therefore devoted to basic but fundamental results. Some of these general facts were known and rediscovered in different contexts [31, 16]. However, they mainly appeared as auxiliary tools in these works. In this paper, we aim at covering the most noteworthy facts in a selfcontained and organized way. In the course of our exposition, we identify which properties of surreal substructures are systematic and which ones are proper to specific structures. We also include a wide range of examples. This effort culminates in the last two sections 7 and 8, where we present the examples that motivated our paper and that are important for our program to construct an isomorphism between and the class of hyperseries. We refer to [5] for some first applications in this direction. In Appendix A, we also compiled a small atlas for the most prominent examples of surreal substructures.
Let us briefly outline the structure of the paper. In section 2, we recall the three main representations of surreal numbers. In section 3, we recall the definitions of basic arithmetic operations on surreal numbers. We also show how to extend the ordinal sum and the ordinal product to .
In section 4, we introduce surreal substructures, our main object of study, as isomorphic copies of inside itself. Any surreal substructure comes with a defining isomorphism that is unique and that we consider as a parameterization of the elements in by . Defining isomorphisms and can be composed to form the defining isomorphism of a new surreal structure that we call the imbrication of inside . More generally, we will often switch between the study of surreal substructures and that of their parameterizations. A consequent part of section 4.1 is a reformulation of notions and arguments found in [31, 16, 17]; see Remark 4.8.
In section 5, we investigate the existence of fixed points for the defining isomorphism of a given surreal substructure . More precisely, we give conditions on under which the class of such fixed points is itself a surreal substructure. Determining the class allows us in some cases to compare the defining isomorphisms of two surreal substructures. This task leads us to study surreal substructures which are closed under nonempty, setsized suprema in of chains in . Such a surreal substructure is said closed, and has the following properties:
Corollary 5.14: for an closed surreal substructure , the class is a surreal substructure, and it coincides with , where denotes the fold composition of with itself. A similar result was first proved by Lurie [31, Theorem 8.2]; see Remark 5.15.
Proposition 5.18: for an closed surreal substructure , there is a decreasing sequence of surreal substructures such that for ordinals , we have
and ,
,
,
if is limit,
In fact any wellordered sequence of closed surreal substructures can be similarly “imbricated”, and thus closed surreal substructures can be seen as words in a rich language that captures at the same time the notions of fixed points, imbrications and intersections of surreal substructures. One direct application is a new proof of a theorem by Lemire [29]; see Remark 5.17.
In section 6, we study subclasses whose elements are the simplest representatives of members in a convex partition of a surreal substructure . Under a settheoretic condition on , we prove that this class forms a surreal substructure of (Theorem 6.7) whose parameterization admits a short recursive definition. A weaker version of this theorem was first proved by Lurie [31]; see Remark 6.8. A particularly important special case is when the convex partition is induced by a group action (see section 6.3). We also introduce the notion of a sharp convex partition of a surreal substructure which makes closed within (Theorem 6.14).
Our final sections 7 and 8 concern the application of our results to some prominent examples of specific surreal substructures. This includes the structure of purely infinite surreal numbers of [21], the structure of monomials of [11], the structure of logatomic numbers of [8], the structure of numbers of [27], and various structures of nested monomials, including . Our results about nested monomials in section 8 are analogous to Lemire's work on continued exponential expressions [30], when replacing ordinal exponentiation by traditional exponentiation. The appendix A contains a short overview of the surreal substructures encountered in this paper.
We will systematically use a bold type face to denote classes such as that may not be sets. Given a partially ordered class and subclasses of , we write if for all and . This holds in particular whenever or . For elements of , we write and instead of and . Given more than two subclasses of , we also write whenever for all .
If , we let denote the class of elements with . In the special case when is an ordered monoid, we simply write and .
We use similar notations for nonstrict orders .
Surreal numbers can be represented in three main ways: as sign sequences, as generalized Dedekind cuts, and as generalized power series over . In this section, we briefly recall how this works, and review the specific advantages of each representation. We refer to [11, 21, 16, 15, 32] for more details.
The sign sequence representation is most convenient for the rigourous development of the basic theory of surreal numbers. It was introduced by Gonshor [21, page 3] and we will actually use it to formally define surreal numbers as follows:
Definition
It follows from this definition that is a proper class. Given a surreal number , it is convenient to extend its sign sequence with zeros to a map and still denote this extension by . In other words, we take for all . Given and , we also introduce its restriction to as being the zero padded restriction of the map to : we set for and for .
The first main relation on is its ordering . We define it to be the restriction of the lexicographical ordering on the set of all maps from to . More precisely, given distinct elements , there exists a smallest ordinal with . Then we define if and only if .
The second main relation on is the simplicity relation : given numbers , we say that is simpler than , and write , if . We write for the set of surreal numbers that are strictly simpler than . The partially ordered class is wellfounded, and is wellordered with order type .
Every linearly ordered—and thus wellordered—subset of has a supremum in : for any and , one has ; for any with all , one has . We will only consider suprema in and never in . Numbers that are equal to are called limit numbers; other numbers are called successor numbers. Limit numbers are exactly the numbers whose length is a limit ordinal.
If are sets of surreal numbers satisfying , then there is a simplest surreal number, written , which satisfies [21, Theorem 2.1]. We call the Conway bracket. Notice that is the simplest such number in the strong sense that for all with , we have . The converse implication may fail: see Remark 4.21 below.
Now consider two more sets of surreal numbers with . If has no strict upper bound in and has no strict lower bound in , then we say that is cofinal with respect to . We say that and are mutually cofinal if they are cofinal with respect to one another, in which case it follows that . These definitions naturally extend to pairs of classes with . Note however that is not necessarily defined for such classes. Indeed, there may be no number with (e.g. for and ).
We call a pair of sets with a cut representation of . Such representations are not unique; in particular, we may replace by any mutually cofinal pair . For every surreal number , we denote
which are sets of surreal numbers. We call and the sets of left and right options for . By [21, Theorem 2.8], one has and the pair is called the canonical representation of .
This identity is the fundamental intuition behind Conway's definition of surreal numbers precisely as the simplest numbers lying in the “cut” defined by sets of simpler and previously defined surreal numbers. Of course, this is a highly recursive representation that implicitly relies on transfinite induction.
Conway's cut representation is attractive because it allows for the recursive definition of functions using by wellfounded induction on or its powers. For instance, there is a unique bivariate function such that for all , , we have
(2.1) 
Here we understand that denotes the set and similarly for . This recursive definition is justified by the fact that the elements of the sets , , and are all strictly simpler than for the product order on . This precise equation is actually the one that Conway used to define the addition on . We will recall similar definitions of a few other arithmetic operations in section 3 below.
Let be a field and let be a totally ordered multiplicative group for the ordering . A subset is said to be wellbased if it is wellordered for the opposite ordering of (i.e. there are no infinite chains in ). A wellbased series in and over is a map whose support is a wellbased subset of . Such a series is usually written as , where and the set of all such series is denoted by . Elements in and are respectively called coefficients and monomials. We call the monomial group. The support of any nonzero element admits a largest element for , which is called the dominant monomial of and denoted by .
It was shown by Hahn [22] that forms a field for the natural sum and the usual Cauchy convolution product
In , there is also a natural notion of infinite sums: if is a set and is a family of wellbased series in , then we say that it is summable if is wellbased and is finite for every . In that case, we define the sum of this family by
Consider a second monomial group and a map . We say that is strongly linear if it is linear and for every summable family in , the family is summable in with . By [25, Proposition 10], in order to show that a linear map is strongly linear, it suffices to prove that the above condition holds for families of scalar multiples of monomials. So is strongly linear if and only if for all , the family is summable, with
Since the support of any is wellbased, the order type of for the opposite order of is an ordinal. Now consider an number . We recall that this means that , where stands for Cantor's th ordinal power of . It is known [20, Corollary 6.4] that the series with form a subfield of .
The ordering on induces a natural valuation on whose residue field is . The Archimedean class of a nonzero surreal number is the class of all with the same valuation as . One of the discoveries of Conway was that admits a simplest element that we will denote by . Let be the class of all that we may obtain in this way. Conway also constructed an order preserving bijection that extends Cantor's ordinal exponentiation.
Through this map and the socalled Conway normal form [11, Chapter 5], it turns out that the field is naturally isomorphic to a field of wellbased series , for which becomes the monomial group. For this series representation, any number has a setsized support . The Conway normal form of coincides with its expression as a series . For we sometimes write instead of in order to indicate that we have , and thus that is a truncation of as a series.
In the sequel of this paper, by “number”, we will always mean “surreal number”.
We already explained the usefulness of Conway's cut representation for the recursive definition of functions on and mentioned the addition (2.1) as an example. In fact, one may define all basic ring operations in a similar way:
One major discovery of Conway was that the surreal numbers actually form a real closed field for these operations and the ordering . As an ordered field, it naturally contains the dyadic numbers, which are the numbers with finite length, and the real numbers, which are the numbers of length whose sign sequence does not end with infinitely many consecutive identical signs.
The class of ordinals is also naturally embedded into by identifying an ordinal with the constant sequence of length with for all . Thus, in , expressions such as
make sense and are amenable to various computations and comparisons. See [11, Chapter 1] for more details on the field operations on . See [21, Chapters 1, 2 and 3] for more details on those operation in the framework of sign sequences and on the correspondence between cuts and sign sequences.
Using hints from Kruskal, Gonshor also defined an exponential function on , which we denote by [21, Page 145]. This function extends the usual exponential function on . In fact, it turns out that is an elementary extension of as an ordered exponential field [12, Corollary 5.5]. In other words, the usual exponential function and its extended version to satisfy the same first order properties over .
In order to define for using a recursive equation, one needs to find an appropriate characterization of the cut formed by inside the field generated by , , and . In exponential fields, the natural inequalities satisfied by such cuts involve truncated Taylor series expansions. Given and , let
If and is such that is already defined, then for , we should have
and one expects that such inequalities give sharp approximations of . Following this line of thought, Gonshor defined
The reciprocal of , defined on , is denoted . This also leads to a natural powering operation: given and , we define . Given , we have , but for more general elements , one does not necessarily have . (see [6] for more details).
We write and for the classes of nonzero and limit ordinal numbers, respectively. The class of ordinal numbers is equipped with two distinct sets of operations: Cantor's (noncommutative) ordinal arithmetic and Hessenberg's (commutative) arithmetic. For ordinals , we will denote their ordinal sum, product, and exponentiation by , and . Their Hessenberg sum and product coincide with their sum and product when seen as surreal numbers [21, Theorems 4.5 and 4.6]; accordingly, we denote them by and . We assume that the reader is familiar with elementary computations in ordinal arithmetic. In this section, we define operations on surreal numbers which extend ordinal arithmetic.
For numbers , we let denote the number, called the concatenation sum of and , whose sign sequence is the concatenation of that of at the end of that of . So is the number of length , which satisfies
(α<ℓ(x))  
(β<ℓ(y)) 
It is easy to check that this extends the definition of ordinal sums. Moreover, the concatenation sum is associative and satisfies whenever and is a limit number.
We let denote the number of length , called the concatenation product of and , whose sign sequence is defined by
(α<ℓ(y),β<ℓ(x)). 
Here we consider as a product in . Informally speaking, given and , the number is the fold rightconcatenation of with itself, whereas is the number obtained from by replacing each sign times by itself. We note that extends Cantor's ordinal product.
The operations and will be useful in what follows for the construction of simple yet interesting examples of surreal substructures. The remainder of this section is devoted to the collection of basic properties of these operations. The proofs can be skipped at a first reading, but we included them here for completeness and because we could not find them in the literature. We refer to [11, First Part] for a different extension of the ordinal product to the class of games (which properly contains ).
Proof. ) Both and have length . Let and . Write where and . Then
) The numbers and have length . For , we have and .
) The number has length
Let and . If , then
Otherwise, there is such that and then
) The previous identities imply in particular that is linearly ordered by simplicity, which means that the supremum is well defined in . Assume is limit. If , then we have . Assume . Notice that we have , so
Let and . Since is a limit number, there is such that . Then
Remark
(3.7) 
Likewise, the contatenation sum has the following equation [15, Proposition 2]:
(3.8) 
Note that these two equations are not uniform in the sense of Definition 4.29 below.
If , then if and only if .
If , then if and only if .
Proof. a) If , then for with , Lemma 3.1(c) implies that
Conversely, if , then since , we may compute, for , the sign . We deduce that , so .
b) If , then given the maximal common initial segment of and , we have , with . Thus is strictly smaller than , which means that . Since the order is linear, this suffices to prove the result.
Let be a subclass of and let be a family of ordering relations on . Then we say that a function is increasing if is increasing for each with . If is also injective, then we say that it is strictly increasing. If we have for all and , then we call an embedding of into . We simply say that is an embedding if is a embedding.
Definition
Example
For , the map gives rise to the surreal substructure of numbers whose sign sequences begin with the sign sequence of .
For , the map induces the surreal substructure of numbers whose sign sequences are (possibly empty or transfinite) concatenations of the sign sequences of and .
Example
We claim that any strictly increasing map is automatically an embedding. We first need a lemma.
Lemma
Proof. Since , we have if and only if there is with and . Now so and likewise holds if and only if there is with and . Notice that and imply that . In both cases, since , we have and . Therefore the existence of yields that of and vice versa. The other equivalence follows by symmetry.
Lemma
Proof. Since is a linear order, the function is automatically an embedding for , so we need only prove that it is an embedding for . Assume for contradiction that there are elements of such that and . Let be the maximal common initial segment of and . We have , so . Since is strictly increasing, we have and , which given our assumption contradicts the previous lemma. Hence , which concludes the proof.
Since a surreal substructure is an isomorphic copy of into itself, it should induce a natural Conway bracket on . This actually leads to an equivalent definition of surreal substructures. Let us investigate this in more detail.
Let be an arbitrary subclass of . We say that is rooted if it admits a simplest element, called its root, and which we denote by . Given subclasses of , we let denote the class of elements such that . If is rooted, then we let denote its root. If and are sets, then we call the cut in defined by and . If for any subsets of the class is rooted, then we say that admits an induced Conway bracket.
Proposition
is an isomorphism .
Proof. We first justify that is well defined. Let be such that is welldefined and strictly increasing on , with values in . We have where those sets are in so is a welldefined element of , and is strictly increasing on . By induction, is a strictly increasing map . Let with , so that . By definition, the number is the simplest element with . Since and , it follows that . We deduce from Lemma 4.5 that is an embedding of into itself.
We now prove that by induction on for . Let be such that is a subset of . Let and where since is strictly increasing and thus injective, the sets are uniquely determined and satisfy . Since admits an induced Conway bracket, the cut is rooted and contains , so . Since , we necessarily have . By induction, we conclude that .
Proposition
Proof. Assume that admits an induced Conway bracket. By the previous proposition, is the range of the strictly increasing function , whence is a surreal substructure. Conversely, consider an embedding of into itself with image . Let be subsets of and define . The function is strictly increasing so , and we may consider the number . Now let . We have , so . Since is increasing, this implies , which proves that , so admits an induced Conway bracket.
Remark
For all subsets of with , the class is either empty or rooted.
A subclass satisfies IN if and only if there is a (unique) initial subclass of and a (unique) isomorphism . This is in particular the case for the classes described in Section 6 below. For more details on this more general kind of subclasses, we refer to [16].
In this paper, we focus on surreal substructures. The characterizations given in Proposition 4.7 and Proposition 4.13 are known results. The second one was first proved (for more general types of ordinal sequences) by Lurie [31, Theorem 8.3], and both of them were proved by Ehrlich [16, Theorems 1 and 4].
Proposition
Proof. Let be a strictly increasing function with image . By Lemma 4.5, it is an embedding. Given such that and coincide on , the numbers and of are both the simplest element of and are thus equal. It follows by induction that .
Lemma
Proof. By Proposition 4.6, the map realizes an embedding of into , so the order type of the former is smaller than that of the latter, namely .
Given a surreal substructure , we call the defining surreal isomorphism of parametrization of . The above uniqueness property is fundamental; it allows us in particular to perform constructions on surreal substructures via their defining surreal isomorphisms and vice versa.
Let be a surreal substructure. Given an element and subsets of with , we say that is a cut representation of in if . We refer to elements in and as left and right options of the representation. For , we write
and call this pair the canonical representation of in . We also write for the set .
A final substructure of is a rooted final segment of for (and thereby necessarily a substructure). It is easy to see that this is the case if and only if is rooted and is the class of elements such that .
Proposition
Proof. The assertions ) and ) are true when by [21, Theorems 2.5 and 2.9]. By Proposition 4.6, the function is an isomorphism , satisfying the relation , so ) and ) hold in general. We have , since . Conversely, for and , we have and , so and have the same sign. We conclude that , which completes the proof of ).
Let , be two surreal substructures. Then there is a unique isomorphism that we call the surreal isomorphism between and . The composition is also an embedding, so its image is again a surreal substructure that we call the imbrication of into . We say that is a left factor (resp. right factor) of if there is a surreal substructure such that (resp. ).
By the associativity of the composition of functions, the imbrication of surreal substructures is associative. Right factors are determined by the two other substructures. More precisely, since is injective, the relation yields . The same does not hold for left factors:
Proposition
Proof. If , then . Assume that and let . We have where and , are respectively embeddings and so is an embedding . Hence is a surreal substructure with , which means that .
Through the identification , the class of surreal numbers can naturally be represented by a full binary tree of uniform depth , as illustrated in Figure 4.1 .
For each ordinal , we let denote the subtree of of nodes of depth , that is, the set of numbers with . This can be represented as the subtree obtained by cropping the picture at depth . In order to characterize surreal substructures in treetheoretic terms, we need to investigate chains for : given a subclass , a chain in is a linearly ordered (and thus wellordered) subset of . If a chain in admits a supremum in , we denote it . Note that the empty set has a supremum in if and only if has a root, in which case . We say that is the left successor of if and for every in . Right successors are defined similarly.
Proposition
is a surreal substructure.
Every element of has a left and a right successor in and every chain in has a supremum in .
Proof. Let be a surreal substructure. In , any element clearly admits a left successor and a right successor , and every chain clearly admits a supremum. Since these properties are preserved by the isomorphism , we deduce ).
Assume now that ) holds. We derive ) by inductively defining an isomorphism . Applying ) to the empty chain, we note that the supremum of in is the minimum of for . So is rooted and we may define . Let be an ordinal such that is defined and strictly increasing on . We distinguish two cases:
If is limit, then let be a surreal number with length . Thus is a limit number and is a chain in . We define .
Assume now that is successor, let be a number with length , and write where . Let and be the left and right successors of . Then we define .
In both cases, this defines on and the extension is clearly strictly increasing and strictly increasing on every set for .
It remains to be shown that is strictly increasing on . Given in , let be their maximal common initial segment. We either have and thus , or and thus . So is strictly increasing on .
By induction, the function is defined and increasing on . Note that is wellfounded since is wellfounded and . By induction over , let us show that lies in the range of . If is the left or right successor of an element , then the induction hypothesis implies the existence of some with , and we get . Otherwise, we have where . We conclude that is an isomorphism.
Example
Example
The characterization ) gives us some freedom in constructing a surreal substructure: one only has to provide a mechanism for chosing left and right successors of already constructed elements, as well as least upper bounds for already constructed branches (i.e. chains). Intuitively speaking, this corresponds to a way to “draw” as a full binary tree inside the binary tree that represents : see Figure 4.2 .
If are subclasses of , recall that is convex in if
We simply say that is convex (resp. convex) if it is convex (resp. convex) in . We let denote the convex hull of in , that is, for every number , we have if and only if and there are elements of such that . The convex hull of in is the smallest convex subclass of containing .
Lemma
Proof. In view of Propositions 4.6 and 4.7, it suffices to prove the lemma for . Let be a nonempty convex subclass of . Assume for contradiction that are two simplest elements with . Let be the smallest ordinal such that . Since and , we must have and . Now consider the number whose sign sequence is . Then , whence , but also ; a contradiction.
Lemma
Proof. Given , we have , so C contains an ordinal. Let denote the smallest ordinal in . Given another ordinal , we have by minimality of . Since C is a final segment of No, it follows that . For any , we deduce that lies in the cut , whence . This shows that .
Proposition
A convex subclass of is a surreal substructure if and only if it has no cofinal or coinitial subset.
If is a surreal substructure, then is a surreal substructure.
If is a surreal substructure, is a cut in and is strictly monotonic and surjective, then is a surreal substructure.
The intersection of any setsized decreasing family of surreal substructures that are convex in is a surreal substructure.
Proof. a) Assume that has no cofinal or coinitial subset and let be subsets of .
If both and are empty, then for any . Notice that , since is not cofinal in .
If and , then there exists an with , since is not coinitial in . Let and . Then , so , and .
Similarly, if and , then for some in .
If and , then , by convexity.
In each of the above cases, we have shown that is a nonempty convex subclass of . By Lemma 4.16, it is rooted. By Proposition 4.7, it follows that is a surreal substructure. Conversely, if is a surreal substructure, then given a subset of , we have
so is neither cofinal nor coinitial in .
b) This is a direct consequence of the previous point: the cut is by definition a convex subclass of , and given a subset of we have
By Proposition 4.7, it follows that is a surreal substructure.
c) Since is a surreal substructure, it has no cofinal or coinitial subset. It follows that the same holds for , which is thus a surreal substructure.
) We have is is increasing and if is decreasing. In both cases, is a cut in , hence a surreal substructure by ).
) Let be a linearly ordered set and let be decreasing for . Its intersection is convex. Let be a subset of . For , we have whence where . and . Writing and , we have . Moreover, for , we have so by convexity. This proves that and consequently that is neither cofinal nor coinitial in . Therefore is a surreal substucture by ).
Example
Example
The class of strictly positive surreal numbers is a convex surreal substructure, and it is in fact the final substructure of .
Likewise, the class of positive infinite surreal numbers is a convex surreal substructure.
The class of infinitesimals forms a surreal substructure which can be split as the union of and the two final substructures , .
Although every interval for is a convex surreal substructure, their increasing union is not a surreal substructure.
Remark
Any convex subclass of is a generalized cut in where is the class of strict lower bounds of in and is the class of its strict upper bounds. However, those classes may not always be replaced by sets. In fact, the class is a cut with subsets of if and only if such sets can be found that are mutually cofinal with . The existence thus amounts to since cofinality is invariant under mutual cofinality (see the end of Appendix B for notes about cofinal wellordered subsets).
Example
We already noted that the Conway bracket allows for elegant recursive definitions of functions on . Let us now study such definitions in more detail and examine how they generalize to arbitrary surreal substructures.
Definition
We say that the cut equation is extensive if it satisfies
Note. We will see in the proof of Proposition 4.27 below that extensive cut equations preserve simplicity.
Example
Taking and , and , we obtain the function with for all and for all .
See Example 4.32 below for more examples.
Remark
Example
are extensive. We will write in this case. Note that it is common to consider welldefined cut equations of the form
where itself belongs to and .
Proposition
Proof. We claim that is increasing. Indeed, let with . We have , so and . We deduce by extensivity of that and , and thus . This implies that . Thus is strictly increasing. So the composition is strictly increasing. The function is an embedding by Proposition 4.6, so embeds into . In particular, is a surreal substructure. By Proposition 4.9, we conclude that .
As an application, we get the following wellknown result (see [8, Proposition 4.22]).
Proposition
Proof. We have . By Proposition 4.18(b), this is a surreal substructure. Recall that for , we have . If , then we have so we may write
Seen as a cut equation in , this is an extensive cut equation, so by Proposition 4.27, we see that is a surreal substructure and that realizes the isomorphism .
Definition
whenever is a cut representation of in . We say that is uniform if it is uniform at every .
Example
is uniform. On the contrary, the following cut equation for is not uniform:
Indeed, we have and , but .
Example
which is uniform. On the contrary, the cut equation for is not uniform:
Indeed, if we were to apply this cut equation to the cut presentation of , then we would have as a left option and as a right option, which cannot be.
Example
Example
(4.1) 
Similarily for the multiplication, we have
where , , and range in , , and respectively.
Uniform cut equations have the interesting property that they can be composed.
Lemma
Then has the uniform cut equation where for every cut representation in , we have and .
Proof. Let , let be a cut representation of in . By uniformity of the cut equation of at , we have
By uniformity of the cut equation of at , we have
whence the result.
Recall that a class is cofinal (resp. coinitial) with respect to a class if every element of has an upper bound (resp. lower bound) in . If , then we simply say that is cofinal (resp. coinitial) in .
Lemma
Proof. Let us first prove uniformity in the case when . Let be sets of surreal numbers and let . Since is strictly increasing and ranges in , the number is well defined and , which yields . Moreover, the set is cofinal in whereas is coinitial in , so . Hence and , which shows that the cut equation is uniform.
Now consider the general case and let be subsets of . Setting and , we have by uniformity of the cut equation for . Furthermore,
by uniformity of the cut equation for . Hence , which proves that is uniform. This cut equation has the form where are sets of functions, so it is extensive.
The above proposition shows that surreal isomorphisms satisfy natural extensive cut equations. Inversily, Proposition 4.27 shows that extensive cut equations give rise to surreal isomorphisms. As an application, if we admit that the operation
is well defined, then we see that it defines a surreal isomorphism. This is the parametrization of the class of monomials, that is, Conway's map. This cut equation is also uniform (see [21, corollary of Theorem 5.2]), and we can for instance compute, for every number , the number
Whenever they exist, this shows the usefulness of extensive cut equations. Unfortunately, many common surreal functions such as the exponential do not admit extensive cut equations. The next proposition describes a more general type of cut equation that is sometimes useful.
Proposition
Then induces an embedding for each element of .
Proof. Let . If and satisfies , then is cofinal with respect to and hence to , and is cofinal with respect to and hence to , so . Therefore is a nonempty convex subclass of . Note that for , we have
For numbers lying in with , we have , which implies that . Since is a nonempty convex subclass of and is increasing and bijective, the class is a nonempty convex subclass of on which is strictly increasing. By Lemma 4.5, the function induces an embedding and thus induces an embedding .
Example
Here we have and .
After introducing the map as a way to parameterize the class of monomials, Conway remarks that for any ordinal , the number coincides with Cantor's th ordinal power of . He then goes on with the definition of generalized numbers as surreal numbers such that . It turns out that the class of generalized numbers can be parameterized as well and actually forms a surreal substructure: see Conway's informal discussion [11, p 34–35] and Gonshor's formal proof [21, Theorem 9.1 and Corollary 9.2]. Gonshor gives further conditions for the class of fixed points of a surreal function to be a surreal substructure [21, Theorem 9.4].
In this section, we consider the more general problem of deciding, given a surreal substructure , whether admits fixed points, and possibly a whole surreal substructure of fixed points. A related fixed point theorem was obtained by Lurie [31, Theorem 8.2] in a somewhat different context.
For operators where are subclasses of and , it will be convenient to write for the fold composition of with itself. In particular, .
Definition
If are surreal substructures with , then for every number , we have if and only if , and if and only if . In particular, the parametrizations of and coincide exactly on .
Proposition
Proof. Let . For , we have , so . Assume for contradiction that is a proper subclass of , and consider with minimal length. For , let with . For all , we have , so by our minimality assumption and Lemma 4.10, we have .
Recall that is not fixed, so . By symmetry, we may assume without loss of generality that , which implies that for all . For , let be the maximal element of with . This element is welldefined since is a surreal substructure and . The number is maximal in with , whence , so .
Since and , we have and . We deduce that and that . In particular, we have so , so is not fixed, and we have .
Since for each , Lemma 4.10 implies . The latter decreasing sequence of ordinals is necessarily stationary; let be such that for all . By Lemma 4.10, it follows that for all , whence . But , which contradicts the minimality of . This absurdity completes our proof.
Example
If is the final substructure , then for any surreal number , the sign sequence of is obtained through concatenation of the sign sequences of and . Thus fixed numbers are numbers whose sign sequences start with copies of the sign sequence of , that is .
Consider where is a strictly positive number. Let and for . We claim that where .
Indeed, since and is a surreal isomorphism, we have for every , so is well defined. We have . For every number where , we have , so . Conversely if , then so , so is equal to for some ordinal . For , , so . Let denote the number of length defined at the level of sign sequences by
We claim that . Indeed, for and , there is such that , and we have
Thus , so .
We let denote the surreal substructure which is the class of surreal numbers, whose sign sequence contains no consecutive distinct signs. Elements in are called purely infinite numbers, since their supports as series contains only infinitely large monomials: see Proposition 7.4 below.
As mentioned at the beginning of this section, if is the class of monomials, then is the map , and its fixed points are called generalized numbers. For , the number is usually denoted , and the map extends the parametrization of numbers in . We refer to [21, Chapter 9] for a detailed study.
If (where ), then for , we have
Consider the function . For all and , we have by [21, Theorem 5.12]. Recall that . Thus for , we have
So and coincide on . Since and the class of fixed points of are contained in , we deduce that is the class of fixed points of .
Now, informally speaking, we would like to consider the expression
as a notation for “the” fixed point of . However, this expression is inherently ambiguous, since actually contains many elements. The map can be regarded as a notation to provide an unambiguous expression for each fixed point , using a single surreal parameter with . In a similar manner, one may regard the notation as a way to disambiguate
If is the interval , then we can see that fixes pointwise and replaces the initial segment (resp. ) in the sign sequence of a positive (resp. negative) infinite number with (resp. ). Since , we deduce that the defining isomorphism fixes , , and pointwise. One can check that the class
is a surreal substructure.
In general, the class may not be a surreal substructure. For instance, the class defined in Example4.14 satisfies , and consequently has no fixed point. This raises the question of finding a condition on that will ensure to be a surreal substructure. One obvious first idea is to investigate when decreasing intersections of surreal substructures are surreal substructures.
We introduce a notion of closed subclasses of an ambient surreal substructure . In the case when is a surreal substructure, we characterize its closedness in terms of its defining surreal isomorphism.
Definition
The intervals and are closed convex surreal substructures. The interval is a surreal substructure which is not closed, since .
The structure introduced in Example 5.3 is a nonconvex closed surreal substructure since having no different consecutive signs in one's sign sequence is preserved by taking suprema in .
Likewise, the structure is closed.
If is a surreal substructure defined by the tree construction (see Proposition 4.13), then it is closed if and only if for each nonempty chain in , the element of is defined as . In particular, the surreal substructure from Example 4.14 is not closed.
The class is closed but has a proper class of minimal elements (in particular, it has no root).
The term “closed” suggests the existence of a topology. Indeed, we have:
Proposition
Proof. It is clear that and are closed. Let be the intersection of a (possibly proper classsized) nonempty family of closed subclasses of . Let be a nonempty chain in . We have for all , whence and is closed.
Let be closed subclasses of and let be a nonempty chain in . If admits a maximum, then . Otherwise, let be such that is cofinal in . Then , so is closed.
Lemma
Proof. The class is final in , thus suprema of nonempty chains in lie in .
It will sometimes be useful to comprehend closure in terms of projections.
Proposition
Proof. Assume that is closed. Consider with . Then the set of initial segments of lying in is nonempty and closed under taking suprema in . Consequently, indeed admits a maximal initial segment in . Inversely, assume that is well defined on and let be a nonempty chain in . If has a maximum, then . Otherwise, , so . This shows that is closed.
Definition
Since is increasing when it exists, its fibers are convex in .
Lemma
Proof. Let . Since , we have , whence . The class is closed so has a maximal initial segment lying in . Now is an initial segment of lying in , whence . We may thus consider the maximal initial segment of that lies in . If is simpler than , then , since . Similarly, , since . This proves that is the maximal initial segment of lying in .
We will mostly consider closures of surreal substructures in other ones. In this situation, closure can be regarded as a property of the defining surreal isomorphism:
Lemma
Proof. Assume that the relation holds. Let be a nonempty chain in and consider the set . Since is an embedding, the set is a nonempty chain in , whence (see Proposition 4.13). Our assumption on gives , so , and is closed. Conversely, assume is closed. Let be a nonempty chain. Since is increasing, the set is a nonempty chain in , so , whence , which is the desired equality.
Lemma
If , then is closed if and only if sends closed subclasses of onto closed subclasses of .
If and are closed, then so is .
If and are closed, then so is .
Proof. ) Assume is closed and is a closed subclass of . Let be a nonempty chain in . The set is a nonempty chain in so its supremum lies in , and , so is closed. Conversely, if sends closed classes of surreal numbers onto closed subclasses of , then in particular is closed.
) This is a direct consequence of ).
) Assume that and are closed. Let be a nonempty chain in . Then , and since is injective, we get , so is closed.
We now come to the main interest of the notion of closure.
Proposition
Proof. We use the characterization of surreal substructures given in Proposition 4.13. By Proposition 5.6, the class is closed. In particular, the class has suprema of nonempty chains. We also have which lies in by the closure of each structure for , so the empty chain has a supremum as well.
Let us now treat the case of left and right successors. Given , let and be the left and right successors of in , for each ordinal . For , we have and , so by the definition of left successors. Similarly, we get . Thus the sets and are chains whose suprema in satisfy . For with and , we have so , whence . This means that is the right successor of in . Likewise, is the left successor of in . We conclude that is a surreal subtructure.
Corollary
Proof. This is a direct consequence of Lemma 5.12, Proposition 5.13 and Proposition 5.2.
Remark
is a good tree, but not a surreal substructure, since has two right successors and two left successors.
The class of closed surreal substructures being closed under decreasing intersections, we are now in a position to define a notion of transfinite rightimbrications of closed surreal substructures.
Theorem
.
if ,
if is limit.
Then each class for is an closed surreal substructure, and if , then we have
Proof. We first need to prove that the definition is warranted. We do this by transfinite induction, while proving at the same time that the sequence is decreasing, and that each term is an closed surreal substructure. Let be such that these assumptions hold strictly below . If is a successor ordinal, then and are closed surreal substructures, whence is well defined and closed (by Lemma 5.12). The surreal substructure is a left factor of , which implies that . If is limit, the intersection that defines is an closed surreal substructure by Proposition 5.13, and is clearly decreasing.
We prove the identity (5.1) by induction on . Let be an ordinal such that (5.1) holds for any sequence and with . Let be such that . If for some ordinal , then
If is limit, then we have
(The injectivity of allowed us to move it through intersections).
Example
He outlined an approach for proving that the class of numbers that can be expressed in this way is order isomorphic to . Conway's ideas were rigorously worked out by Lemire [28, 29, 30]. He first proved the following result in the case when : given , let be the class of numbers such that there exists a sequence with
for all . Then is order isomorphic to . Moreover, writing for the isomorphism, has fixed points of any order , and the class of such fixed points is also order isomorphic to . This result follows from Theorem 5.16 by taking for all and . Then for all .
A similar result was proved by Lemire for more general continued exponential expressions [29, Theorem 4]. This result is more involved and presents similarities with our results about nested expansions in section 8 below.
Proposition
Each is an closed surreal substructure, and for , we have:
Proof. Most of this is a direct consequence of Theorem 5.16; we only need to prove the identity (5.3). Let be such that this identity holds for . Let be ordinal numbers with . Corollary 5.14 justifies that the same construction can be applied to the structure . If for , then we have
where we used (5.2) as well as the inductive hypothesis. If is limit, then
Note that for , the structure is the fold imbrication of into itself, and we have . For , we have , by Proposition 5.2 and the identity (5.3). Thus transfinite rightimbrications of with itself allow us to define higher order fixed points of as being elements of the with . As we have seen, imbrication is leftdistributive on decreasing intersections that form a surreal substructure. It is not rightdistributive in general. For instance if is a proper closed surreal substructure of , then is a proper subclass of .
Example
Example
Throughout this section, stands for a surreal substructure.
Definition
Remark
We can obtain as through the discrete partition with for all . Let for all . The map is a surjective, increasing projection. We refer to it as the simple projection.
For the remainder of this subsection, let be a convex partition of . A quasiorder (or preorder) is a binary relation that is reflexive and transitive. The following lemma states basic facts on partitions of a linear order into convex subclasses.
Lemma
Proof. It is well known that the partition corresponds to the equivalence relation on . The transitivity and irreflexivity of follow from that of on subclasses of . That its restriction to is a linear order is a direct consequence of the definition of and the equivalence stated above, which we now prove. If has only one member, then the result is trivial. Else let with . We have so . Conversely, assume that . Then which since is a partition implies that . For , there may be no element of such that for this would imply whence by convexity of this class: a contradiction. We thus have , that is, . By definition of , the relation implies that , whereas implies that , so , so .
For any subclass of , we let denote the class .
Lemma
.
.
.
Proof. All inequalities are vacuously true if or . Assume that and are nonempty and let and . Assume for contradiction that , but . Then there exist and with . By convexity of , this yields , whence . This contradiction shows that . The inverse implication clearly holds. The equivalence holds for similar reasons.
Lemma
is simple.
There is a cut representation of in such that .
.
Proof. Since is a cut representation of in , the assertion ) implies ).
Conversely, if is a cut representation of in with , then we have by the previous lemma. By Proposition 4.11(b), the cut representation is cofinal with respect to , so . Hence , again by Lemma 6.4. This shows that ) implies ).
Assume now that is simple and let us prove ). For , we have , so , whence . We do not have since , so Lemma 6.3 yields , and in particular . This proves that , and similar arguments yield .
Assume finally that ) holds and let us prove a). We have so . Now the class is neither strictly greater nor strictly lower than , so our assumption imposes . We conclude that is simple.
An order on a set is said to be dense if for any with , there exists a with .
Proposition
Proof. For , let denote the class of elements of such that no simple element lies strictly between and . The definition of the family only depends on the class , and not specifically on . For , we have .
Conversely, let , and assume for contradiction that lies outside of , say . Then and, being dense, there exists a simple element between and . But implies , which contradicts the assumption that there is no simple element between and . We conclude that , which entails in particular that the partition is uniquely determined by .
If is dense, then we call the defining partition of . Notice that this is in particular the case when is a surreal substructure. We next consider a settheoretic condition under which is always a surreal substructure.
We say that is thin if each member of has a cofinal and coinitial subset. For instance, the convex partition of where
is thin. Indeed each class for admits the cofinal and coinitial subset . See Example 6.15 below for more (counter)examples of thin convex partitions. If is thin, then we may pick a distinguished family such that each for is a cofinal and coinitial subset of , with . We write for any subclass of .
Theorem
Proof. Let be subsets of . For and , we have by Lemma 6.3. Therefore holds as well, which means that is well defined. Given and , there exists an with , since is cofinal in . It follows that , whence . A similar reasoning shows that for any . By Lemma 6.5, it follows that is simple. Let be simple. Given and , the simplicity of , , and implies that , and in particular that . We deduce that , so . By Proposition 4.7, we conclude that the class is a surreal substructure.
Remark
This condition is equivalent to the condition that be sharp in our terminology (see below); it fails for the partition of such that
which is the defining convex partition of the set of logatomic numbers. Indeed, we have , where , but
Still, is a surreal substructure and even an closed one.
When is thin, the structure is in addition cofinal and coinitial in , since for , we have . By the previous proposition, we may say that is thin if its defining partition is thin. If is not thin, then may fail to be a surreal substructure, but one can prove that there exists a unique initial subclass of and a unique isomorphism between and .
For instance, we can obtain the ring of omnific integers of [11, Chapter 5] as where for each number , we set . This is not a surreal substructure since the cut is empty. Nevertheless, is initial in . Note that different partitions may yield the same class (for instance replacing and with and respectively and leaving the other classes unchanged), in contrast to the case of dense partitions from Proposition 6.6. The partition in Example 6.15 below is not thin and yet is a surreal substructure.
Proposition
Proof. The cut equation follows from Theorem 6.7 and the relation
Now towards uniformity, consider a cut representation of a number . We have so the number is well defined. Since is cofinal with respect to and is strictly increasing, the number lies in the cut , so . Conversely, we have , so . Since , we have , whence . We conclude that .
Corollary
Proof. If is an ordinal, then is a nonempty final segment of and thus of , so by Lemma 4.17, its simplest element is an ordinal.
For convex partitions of , we write if we have for every , and say that is finer than . If , then .
Recall that a directed set is a partial order such that for all , there exists a with .
Proposition
Proof. Given , the class is a nonempty convex subclass of and . Let be such that and let . Since is directed, there exists a in such that , whence . In particular, and . Since this is true for any , it follows that , so defines a convex partition of .
For , we have if and only if holds for all , so Lemma 6.5 implies . Now for , the set is cofinal and coinitial in , so is thin. Theorem 6.7 therefore implies that the class is a surreal substructure.
Proposition
Proof. We prove the first inequality by induction on . Assuming that the inequality holds strictly below , we have
For , we have where , so , whence in particular . By Proposition 4.11(a), we have , whence the result by induction.
The second inequality is a consequence of the first one in the case when is the discrete partition of , which is minimal and for which . Since is a final segment of , Proposition 4.17 gives . Moreover, for all with , we have , which yields by induction.
We have encountered two different types of projections for surreal substructures. Given an closed rooted subclass of a surreal substructure , the topological projection sends every element to the maximal initial segment of lying in . Given a convex partition of the surreal substructure , the simple projection sends to the unique simple element lying in . It is natural to ask whether both types of projections relate to each other.
Given a surreal substructure and an closed rooted subclass with , the topological projection is defined everywhere on . For each , we define . It is easy to see that defines a partition of into nonempty rooted convex subclasses, and that is the class of roots where ranges in . The members of are not necessarily convex in . For instance, one can prove that the structure is a closed surreal substructure, with , for which contains and but not .
Conversely, given a convex partition of , the class may not be closed, and when it is, it may be that and disagree. In some interesting cases, the projections and do coincide, and has additional properties, as we shall see now.
Definition
Assume that is thin and sharp. Then each element admits the cut representation in . By Proposition 4.11(b), this cut respresentation is mutually cofinal with . In view of Remark 4.21, we thus see that the sharpness is equivalent to the fact that the cut coincides with the final substructure of for every . This corresponds to the notion of simple representation of [8, Definition 2.2]. We say that is sharp in if its defining partition is sharp.
The main interest of sharpness lies in the following equivalences:
Theorem
is sharp.
is closed and .
is increasing.
is closed and is increasing.
Proof. Assume that is sharp. Let us prove ), ) and ). Note that is simple, whence . We know that when it exists is increasing, and that is increasing, so we need only prove that is closed and .
Let be such that . We claim that is simpler than no element of . By symmetry, we may assume without loss of generality that . Since and is sharp, the set is cofinal with respect to . Assume for contradiction that we have for some . Let be such that and . Then . By Lemma 6.3, we also have , whence . It follows that , whence : a contradiction.
Since , our claim implies that is the maximal initial segment of any element of lying in , i.e. that is defined on and coincides with on this class. Since the classes cover , we see that is defined on , and . By Proposition 5.8, the structure is closed.
We next prove that ) is a consequence of ). Assume for contradiction that is closed with and that is not sharp. We treat the case when there are such that but has a strict upper bound in . Then , so , and . In particular, , whence : a contradiction. The other case is similar.
Assume next that is increasing. For and such that , we have , so is the maximal simple initial segment of . This means that is closed with topological projection . So ) implies ).
Assume is closed and is increasing. It follows that each fiber of where is convex for . As we have seen in the introduction of this section, we can construe as where for , we have . By Proposition 6.6, we have , so ) implies ). This concludes the proof.
Example
Let denote the partition of where for , we have
This is actually the defining partition of the class of purely infinite surreal numbers, which is sharp, since for , we have and .
Let denote the partition of
w
This is a thin convex partition of whose class of simple elements contains . However, the number is not simple since it lies in . Thus is not closed; a fortiori is not sharp.
Let denote the class . This is a surreal substructure by Proposition 4.18. Let denote the convex partition of where for , we have
One can check that each is a convex subclass of and that for , we have , where is the topological projection . By Theorem 6.14, is sharp, but not thin.
We end this subsection with two further properties of sharpness.
Proposition
Proof. We know by Proposition 6.11 that is a thin convex partition of with . Let . For and , there is such that where and . Since is sharp, there exists an with , so is cofinal with respect to . Likewise is coinitial with respect to , so is sharp.
Proposition
Proof. We already know from Corollary 6.10 that . Let be such that is an ordinal. The set is both empty and coinitial with respect to , which implies that and thus that is an ordinal.
In this subsection, we study one particularly important way in which convex partitions of surreal substructures arise, namely as convex hulls of orbits under a group action.
Let be a fixed surreal substructure. We define to be the (classsized) group of strictly increasing bijections , with functional composition as the group law. Consider any setsized subgroup of . Then naturally acts on through function application; we call a function group acting on .
Definition
Proposition
Proof. Let . For any , we have . Indeed, we have for certain . Given , we also have for certain , whence , so that . We also have , whence and for any . The class is convex by definition. For , we know that contains , so the for form a convex partition of . For , the set is cofinal and coinitial in , so this partition is thin.
We write for the partition from Proposition 6.19 and say that an element of is simple if it is simple. We let denote the class of simple elements. Proposition 6.19 implies that every property from Lemmas 6.3, 6.5 and 6.4 applies to the class of simple elements. We call the simple projection and write , , and instead of , and .
Proposition
Proof. This is a direct consequence of Proposition 6.19, Theorem 6.7 and Proposition 6.9, where we take to be the required cofinal and coinitial subset of for each .
Remark
This relation is transitive and reflexive. If , then , so . If and , then we say that and are mutually pointwise cofinal and we write . In that case, we have .
Let us now specialize Proposition 6.11 to groupinduced convex partitions.
Proposition
Proof. If is simple, then for , we have so is simple. Conversely, assume is simple for all . Then let where for , we have . Since is directed and is increasing, there exists an index with and an element such that for all we have for all , and thus . Since is simple, we have . This yields , so is simple. This proves that .
Proposition
Proof. We have for the same reasons as above. Let . Let us prove by induction on that for , we have . By Lemma 6.5, this will prove that . For , the assertion is immediate. Assume therefore that and decompose , where . For every , we have . Since is simple, the sharpness of implies that there exists an such that . By our inductive hypothesis, we have , so . The inequality is proved similarly.
Remark
We conclude our study of surreal substructures with a closer examination of the action of various common types of function groups. We intentionally introduce these function groups without assigning specific domains; this will allow us to let them act on various surreal substructures.
Given , we define the translation by to be the map
The group acts in particular on and . More generally, if is a setsized subgroup of , then acts on and .
Halos for the action of on are called finite halos and simple elements correspond to purely infinite numbers. The class of purely infinite numbers is sometimes denoted ; see [11, 21].
Given , we define the homothety by the factor to be the map
The group acts in particular on , and . More generally, if is a setsized subgroup of , then acts on , and .
Halos for the action of on are called archimedean classes and simple elements are called monomials. The class of monomials is parameterized by the map and forms a multiplicative cross section that is isomorphic to the value group of as a valued field (the valuation being induced by the ordering). The relations , , correspond to the asymptotic relations , , and from [26, 1]. Given , the projection coincides with the dominant monomial , when considering as a generalized series in .
Given , we define the th power map by
Here and are the exponential and logarithm functions from section 3.1. The group acts in particular on and . More generally, if is a setsized subgroup of , then the group acts on and .
Halos for the action of on are sometimes called multiplicative classes and simple elements fundamental monomials. The class of fundamental monomials is parameterized by the map: see [27, Proposition 2.5].
Writing
for all , we define
Both and act in particular on .
Halos and for the actions of and on are sometimes called levels and logarithmicexponential classes respectively. The simple elements are called logatomic numbers and the class of such numbers is parameterized by the map: see [8, Section 5]. The class of simple elements is denoted by and parameterized by the map: see [27, Section 3].
We notice that each of the above function groups is linearly ordered by
With the exception of , all these groups are also abelian. These are both strong properties which need not be imposed for the material of Section 6.3 to apply.
Throughout this subsection, let be a fixed setsized subgroup of and let . If , then so . If , then given , the set is cofinal with respect to , so , whence .
Proposition
Proof. We already know that is a isomorphism so we only need to prove that it preserves sums. Let be such that preserves sums of elements lexicographically strictly simpler than . Recall that the addition is uniform in the sense that
Applying this to the cut equations given by Proposition 6.9 for , we obtain
and by uniformity of the cut equation for , we get
Thus . By induction, this proves that preserves sums of surreals and consequently that is an additive subgroup of .
Let us now focus on . By induction on , it is easy to see that and for all . In particular, this gives a description of in terms of sign sequences.
Let us next describe the structures for in terms of Conway normal forms and of simplicity for some group acting on . By [12, Corollary 3.1], if is an ordinal, then the set is a subgroup of , which acts by translations on . If , then the sets and are mutually cofinal and coinitial, and , since . We claim that this generalizes to every ordinal.
Proposition
Proof. We proceed by induction on . The result obviously holds for . We saw that it holds for in Example 5.3. Assume that is a successor ordinal. Then the function is additive by Proposition 7.1, so is mutually cofinal and coinitial with . Let be simple. Then is simple, so the inductive hypothesis yields for a certain number . Since , we deduce that . Now for , there is with . We cannot have both and , so the contrapositive of Lemma 4.4 yields . Thus is simple, so . Conversely, for , we have for a certain . We have , so . Similar arguments as above yield , whence . This proves that .
If is a limit ordinal, then Proposition 6.11 yields
A consequence of Propositions 7.1 and 7.2 is that is additive for all . In fact, we even have the following:
Proposition
Proof. Let and . Let us first show that for all and . By Proposition 7.2, the function is additive, so this holds for any dyadic number . In particular we have . Let be a nondyadic real number. Let be such that for all . It is well known that contains only dyadic numbers. By Proposition 7.2 and (3.5), we have
where
The cut equation (3.5) for the surreal product by is uniform [21, Theorem 3.5], so
where
where respectively range in . Let us prove that and are mutually cofinal. Analog relations hold for the other sets so this will yield . Since is additive, for and , we have
Now and by our inductive hypothesis. Moreover, we have , since is dyadic. It follows that
Since is nonzero, we have , so this set is mutually cofinal with the set . Therefore is linear.
Let us next prove by induction that . Let be such that for all . Let be an arbitrary cut representation in such that (resp. ) has no maximum (resp. minimum), so that (resp ) has no minimum (resp. maximum). Then we note that the cut equation
simplifies as
Considering the cut representation of , we deduce that we have
We have seen that is linear, so the induction hypothesis yields
We thus have:
In particular preserves monomials.
Let be a number considered as a series in . By our previous arguments, the number is well defined. For all , we will write and . Let us prove by induction on the order type of that ; this will conclude the proof. The additivity and linearity of yield the result for . If is successor and infinite, then has a minimum and , so
Assume now that is an infinite limit. Since is strictly increasing and monomial preserving, [21, Lemma 5.3] yields
where ranges over . Notice that the left (resp. right) options in the above representation of have no maximum (resp. minimum), so
Our inductive hypothesis yields
This concludes the proof.
Proposition
In particular is a nonunitary subring of , and
Proof. The strong linearity of and the relation give
That this forms a (nonunitary) subring follows from the fact that is closed under addition, whence is closed under multiplication.
In this subsection, is a setsized subgroup of and the defining isomorphism of . We will distinguish between confined and ample subgroups. We say that is confined if it is a subgroup of and ample if not. If is ample, then given , the maximum satisfies , which implies that is cofinal with respect to . Thus on , so . If is confined, then , so . For , natural examples of ample multiplicative subgroups include for , whereas natural examples of confined multiplicative subgroups include .
Remark
Proposition
Proof. We only need to prove that is a morphism . Consider monomials with cut representations and such that , , and likewise for . Then [8, Proposition 4.19] yields
Given , this applies in particular to the cut representation of (and likewise for ) since is ample. We thus have
Note that . Assume and . Since is ample, there exists a such that