X-Git-Url: http://git.subgeniuskitty.com/surreal-numbers/.git/blobdiff_plain/8484c2fd2ad8d6a4bcc64f16142185ad71ffbd5f..420b03029f815fa5f0dbf6a47764ec7e977a88d1:/notes/chapter-1.tex?ds=inline diff --git a/notes/chapter-1.tex b/notes/chapter-1.tex index 265fd80..aa7751c 100644 --- a/notes/chapter-1.tex +++ b/notes/chapter-1.tex @@ -1,4 +1,112 @@ \newpage \section{Notes: Chapter 1} -Hello from chapter 1. +\subsection{Review} + +In the first chapter, Knuth provides two axioms. + +\begin{axiom} \label{ax:number-definition} +Every number corresponds to two sets of previously created numbers, such that +no member of the left set is greater than or equal to any member of the right +set. +\end{axiom} + +For example, if $x = \surreal{X_L}{X_R}$, then $\forall x_L \in X_L$, it must +hold $\forall x_R \in X_R$ that $x_L \ngeq x_R$. + +\begin{axiom} \label{ax:leq-comparison} +One number is less than or equal to another number if and only if no member of +the first number's left set is greater than or equal to the second number, and +no member of the second number's right set is less than or equal to the first +number. +\end{axiom} + +For example, the relation $\surreal{X_L}{X_R} = x \leq y = \surreal{Y_L}{Y_R}$ +holds true if and only if both $X_L \ngeq y$ and $Y_R \nleq x$. + +With no surreal numbers yet in our possession, we construct the first surreal +number using the null set (or void set, as Knuth calls it) as both the left and +right set. Although we have not yet examined its properties, Knuth names this +number ``zero''. Thus, $\surreal{}{} = 0$. + +As his final trick, Knuth defines a second generation of surreal numbers using +$0$ in the left and right set, naming them $1$ and $-1$ and claiming the +following relation. + +$$-1 = \surreal{}{0} \leq 0 = \surreal{}{} \leq 1 = \surreal{0}{}$$ + + +\subsection{Exploration} + +\begin{defi} \label{defi:generation} +A \emph{generation} shall refer to the numbers generated by applying Axiom +\autoref{ax:number-definition} to all extant numbers. Generations are numbered +sequentially such that generation-0 consists of the number $0$, generation-1 +consists of the numbers $-1$ and $1$, etc. +\end{defi} + +Working by hand with Axiom \autoref{ax:number-definition}, generation-2 +contains the numbers shown below. + +$$\surreal{-1}{}, \surreal{1}{}, \surreal{-1,0}{}, \surreal{0,1}{},$$ +$$\surreal{-1,1}{}, \surreal{-1,0,1}{}, \surreal{-1}{1}, \surreal{0}{1},$$ +$$\surreal{-1,0}{1}, \surreal{}{1}, \surreal{-1}{0}, \surreal{}{-1},$$ +$$\surreal{-1}{0,1}, \surreal{}{0,1}, \surreal{}{-1,0}, \surreal{}{-1,1},$$ +$$\surreal{}{-1,0,1}$$ + +\begin{defi} \label{defi:similar} +Two numbers $X$ and $Y$ are \emph{similar} iff, per Axiom +\autoref{ax:leq-comparison}, $X \leq Y$ and $Y \leq X$. This is denoted by $X +\similar Y$. +\end{defi} + +From this point forward, we will refer to similar surreal numbers +interchangeably. + +Using this definition, the twenty numbers from generations 0-2 break down into +ten equivalence classes based on similarity, as shown below. + +$$\surreal{0}{} = \surreal{-1,0}{}$$ +$$\surreal{}{}$$ +$$\surreal{}{0}, \surreal{}{0,1}$$ +$$\surreal{-1}{}$$ +$$\surreal{1}{}, \surreal{0,1}{}, \surreal{-1,1}{}, \surreal{-1,0,1}{}$$ +$$\surreal{-1}{1}$$ +$$\surreal{0}{1}, \surreal{-1,0}{1}$$ +$$\surreal{}{1}$$ +$$\surreal{-1}{0}, \surreal{-1}{0,1}$$ +$$\surreal{}{-1}, \surreal{}{-1,0}, \surreal{}{-1,1}, \surreal{}{-1,0,1}$$ + +From this we see that, since Axiom \autoref{ax:leq-comparison} makes its +comparison element-wise, every surreal number generated by our current methods +must be similar to a surreal number containing one or zero elements in its left +and right sets. This motivates the following definition. + +\begin{defi} \label{defi:reduced-form} +The \emph{reduced form} for a surreal number $X = \surreal{X_L}{X_R}$ is +defined as $\surreal{x_l}{x_r}$ where $x_l$ is the largest element of $X_L$ and +$x_r$ is the smallest element of $X_R$. If either $X_R$ or $X_L$ are the empty +set, then the corresponding $x_r$ or $x_l$ also become the empty set. +\end{defi} + +Note that we are guaranteed largest and smallest elements of the corresponding +non-empty sets from Axiom \autoref{ax:leq-comparison} and our definition of +similarity. We are only building a one-dimensional number line. + + +\subsection{Conjecture} + +If we can build an addition operation which holds for $1 + (-1) = 0$, then we +could start trying to assign meaningful names to some of the elements from +generation-2. It appears that numbers of the form \surreal{n}{} behave like the +number $n+1$ and similarly, \surreal{}{n} behaves like $n-1$. + +If we write a program to generate a bunch of new surreal numbers and graph them +as ``generation vs magnitude'', perhaps we can assign some meaning to numbers +which don't fit the pattern mentioned in the previous paragraph. Maybe these +behave like $1/n$? + +It sort of feels like surreal numbers constructed via finite repetitions of our +current process will end up building something vaguely like the dyadic +rationals. +