Surreal numbers were first proposed by John Conway and later detailed by Donald Knuth in his 1974 book Surreal Numbers: How Two ExStudents Turned on to Pure Mathematics and Found Total Happiness. This book is actually a mathematical novelette, and is notable as perhaps the only time a new mathematical idea has been first presented in a work of fiction. In his book, which takes the form of a dialogue, Knuth coined the term surreal numbers for what Conway had simply called numbers originally. Conway liked the new name, and later adopted it himself. Conway then described the surreal numbers and used them for analyzing games in his 1976 book On Numbers and Games.

The basic idea behind the construction of surreal numbers is similar to Dedekind cuts. We construct new numbers by representing them with two sets of numbers, L and R, that approximate the new number; the set L contains a set of numbers below the new number and the set R contains a set of numbers above the new number. We will write such an approximation as { L  R }. We will pose no restrictions upon L and R except that each of the numbers in L should be smaller than any number in R. For example, { {1, 2}  {5, 8} } is a valid construction of a certain number between 2 and 5. (Which number exactly and why will be explained later on.) The sets are explicitly allowed to be empty. The informal interpretation of a pair { L  {} } will be "a number higher than any number in L", and of { {}  R } "a number lower than any number in R". This leads to the following construction rule:
Given a surreal number x = { X_{L}  X_{R} } the sets X_{L} and X_{R} are called the left set of x and right set of x respectively. To avoid lots of brackets we will write { {a, b, ... }  { x, y, ... } } simply as { a, b, ...  x, y, ... } and { {a}  {} } as { a  } and { {}  {a} } as {  a }.
In order for the generated numbers to actually qualify as numbers there has to be a "less than or equal to" relation (here written as ≤) defined on them. This is supplied by the following rule:
The two rules are recursive, so we need some form of induction to put them to work. An obvious candidiate would be finite induction, i.e., generate all numbers that can be constructed by applying the construction rule a finite number of times, but, as will be explained later on, things get really interesting if we also allow transfinite induction, i.e., apply the rule more often than that. If we want the generated numbers to represent numbers then the ordering that is defined upon them should be a total order. However, the relation ≤ defines only a total preorder, i.e., it is not antisymmetric. To remedy this we define the binary relation == over the generated surreal numbers such that
Since this defines an equivalence relation the ordering on the equivalence classes implied by ≤ will be a total order. The interpretation of this will be that if x and y are in the same equivalence class then they actually represent the same number. The equivalence classes to which x and y belong are denoted as [x] and [y] respectively. So if x and y belong to the same equivalence class then [x] = [y].
Let us now consider some examples and see how they behave under the ordering. The most simple example is of course
which can be constructed without any induction at all. We will call this number 0 and the equivalence class [0] will be written as 0. By applying the construction rule we can consider the following three numbers
The last number is however not a valid surreal number because 0 ≤ 0. If we now consider the ordering of the valid surreal numbers we will see that
where x < y denotes that not(y ≤ x). We will refer to {  0 } and { 0  } as 1 and 1 respectively, and the corresponding equivalence classes as simply 1 and 1, respectively. Since every equivalence class contains only one element we can replace in statements about ordering the surreal numbers with their equivalence classes without the risk of ambiguity. For example, the statement above could also have been written as:
or even
If we apply the construction rule once more we obtain the following ordered set:
We can now make two observations:
The first observation raises the question of the interpretation of these new equivalence classes. Since the informal interpretation of {  1 } is "the number just before 1" we will call it number 2 and denote its equivalence class as 2. For a similar reason we will call { 1  } number 2 and its equivalence class 2. The number { 1  0 } is a number between 1 and 0 and we will call it 1/2 and its equivalence class 1/2. Finally we will call { 0  1 } the number 1/2 and its equivalence class 1/2. More justification for these names will be given once we have defined addition and multiplication.
The second observation raises the question if we can still replace the surreal numbers with their equivalence classes. Fortunately the answer is yes because it can be shown that
where [X] denotes { [x]  x in X }. So the description of the ordered set that was found above can be rewritten to:
which in turn can be rewritten as
The addition and multiplication of surreal numbers are defined by the following three rules:
where X + y = { x + y  x in X } and x + Y = { x + y  y in Y }.
where X = { x  x in X }
where XY = { xy  x in X and y in Y }, Xy = X{y} and xY = {x}Y.
These operations can be shown to be welldefined for surreal numbers, i.e., if they are applied to welldefined surreal numbers then the result will again be a welldefined surreal number, i.e., the left set of the result will be "smaller" than then the right set.
With these rules we can now verify that the chosen names of the numbers we found sofar are appropriate. It holds for instance that 0 + 0 = 0, 1 + 1 = 2 and 1/2 + 1/2 == 1.
The operations as defined above are defined for surreal numbers but we would like to generalize them for the equivalence classes we defined on them. This can be done without ambiguity because it holds that
Finally it can be shown that the generalized operations on the equivalence classes have the desired algebraic properties, i.e., the equivalence classes plus their ordering and the algebraic operations constitute an ordered field, with the caveat that they do not form a set but a proper class, see below. In fact, it is a very special ordered field: the biggest one. Every other ordered field can be embedded in the surreals. (See also the definition of rational numbers and real numbers.)
Until now we have not really looked at what numbers we can and cannot create by applying the construction rule. We will first start with the assumption that we only generate those numbers that can be created by applying the rule a finite number of times. We do this by inductively defining S_{n} with n a natural number as follows:
This leads to the following observations:
As a consequence all generated numbers are dyadic fractions, i.e., can be written as an irreducible fraction
The next step consists of taking S_{ω} and continuing to apply the construction rule to it and thus constructing S_{ω+1}, S_{ω+2} et cetera. Note that the left sets and right sets may now become infinite.
In fact, we can define a set S_{a} for any ordinal number a by transfinite induction. The first time a given surreal number appears in this process is called its birthday. Every surreal number has an ordinal number as its birthday.
Already in S_{ω+1} will we find the fractions that were missing in S_{ω}. For example, the fraction 1/3 can be defined as
Another number that is already constructed in S_{ω+1} is
Next to infinitely small numbers also infinitely big numbers are generated such as
Lots of numbers can be generated this way; in fact so many that no set can hold them all. The surreal numbers, like the ordinal numbers, form a proper class.
The definition of surreal numbers contained one restriction: each element of L must be strictly less than each element of R. If this restriction is dropped we can generate a more general class known as games. All games are constructed according to this rule:
Addition, negation, multiplication, and comparison are all defined the same way for both surreal numbers and games.
Every surreal number is a game, but not all games are surreal numbers. The class of games is more general than the surreals, and has a simpler definition, but lacks some of the nicer properties of surreal numbers. The class of surreal numbers forms a field, but the class of games does not. The surreals have a total order: given any two surreals, they are either equal, or one is greater than the other. The games have only a partial order: there exist pairs of games that are neither equal, greater than, nor less than each other. Each surreal number is either positive, negative, or zero. Each game is either positive, negative, zero, or fuzzy (incomparable with zero). If x, y, and z are surreals, and x=y, then x z=y z. However, if x, y, and z are games, and x=y, then it is not always true that x z=y z.
The surreal numbers were originally motivated by studies of the game Go, and there are numerous connections between popular games and the surreals. In this section, we will use a capitalized Game for the mathematical object {LR}, and the lowercase game for recreational games like Chess or Go.
We consider games with these properties:
For most games, the initial board position gives no great advantage to either player. As the game progresses and one player starts to win, board positions will occur where that player has a clear advantage. For analyzing games, it is useful to associate a Game with every board position. The value of a given position will be the Game {LR}, where L is the set of values of all the positions that can be reached in a single move by Left. Similarly, R is the set of values of all the positions that can be reached in a single move by Right. This simple way to associate Games with games yields a very interesting result. Suppose two perfect players play a game starting with a given position whose associated Game is x. The winner of the game is determined:
Sometimes when a game nears the end, it will decompose into several smaller games that do not interact. For example, in Go, the board will slowly fill up with pieces until there are just a few small islands of empty space where a player can move. Each island is like a separate game of Go, played on a very small board. It would be useful if each subgame could be analyzed separately, and then the results combined to give an analysis of the entire game. This doesn't appear to be easy to do. For example, you might have two subgames where whoever moves first wins, but when they are combined into one big game, it's no longer the first player who wins. Fortunately, there is a way to do this analysis. Just use the following remarkable theorem:
In other words, gluing together several noninteracting games is equivalent to simply adding their Games!
Historically, Conway developed the theory of surreal numbers in the reverse order of how it has been presented here. He was analyzing Go endgames[?], and realized that it would be useful to have some way to combine the analyses of noninteracting subgames. From this he invented the concept of a Game and the addition operator for it. From there he moved on to developing a definition of negation and comparison. Then he noticed that a certain class of Games had interesting properties; this class became the surreal numbers. Finally, he developed the multiplication operator, and proved that the surreals are actually a field, and that it includes both the reals and ordinals. It is amazing that all this came out of the study of Go!
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