[[Conway's On Numbers and Games|Conway's On Numbers and Games]]
📌 Brief Summary
*On Numbers and Games* (ONAG), authored by John Horton Conway in 1976, is a foundational treatise in combinatorial game theory. It introduces the concept of "surreal numbers," an algebraically closed field that encompasses both the real numbers and infinitely large/infinitesimal quantities, constructed through a recursive process of defining numbers via sets of previously defined numbers.

📖 Core Content
* **The Construction of Surreal Numbers:** Conway defines numbers using a recursive rule: a surreal number is a pair of sets $\{L | R\}$, where no element in $L$ is greater than or equal to any element in $R$. This construction begins with the empty set $\{\emptyset | \emptyset\} = 0$, and proceeds through transfinite induction. This method allows for the simultaneous construction of integers, dyadic rationals, real numbers, and infinitesimals (e.s., $\epsilon$) within a single unified framework.
* **Combinatorial Game Theory (CGT) Framework:** Beyond pure number theory, the text establishes the formal definition of a "game" as any structure following the $\{L | R\}$ rule. While all surreal numbers are games, not all games are numbers. A game is considered a "number" only if it satisfies specific conditions regarding the relationship between its Left and Right options (specifically, that no $L \ge R$).
* **Game Values and Equivalence:** The text introduces the concept of game equivalence ($\equiv$). Two games are equivalent if, when added to their additive inverse, they result in a second-level zero game. This allows for the algebraic manipulation of complex game positions (such as those found in games like Hackenbush) using arithmetic operations: addition, subtraction, and multiplication.
* **The Role of Infinitesimals:** A significant contribution of ONAG is the formalization of infinitesimals within a rigorous, non-standard analytic context. By treating $\epsilon$ (an infinitesimal) and $\omega$ (an infinite ordinal) as algebraic entities that obey standard arithmetic laws, Conway provided a toolset for analyzing games that may last an infinite number of moves or possess "fuzzy" values (games that are neither greater than, less than, nor equal to zero).

🔗 Knowledge Connections
* Related Topics: [[Combinatorial Game Theory|Combinatorial Game Theory]], [[Surreal Numbers|Surreal Numbers]], Transfinite Induction, Non-standard Analysis
* Projects/Contexts: The construction of the field of Surreal Numbers, Analysis of impartial and partizan games (e.g., Hackenbush, Go, Chess)
* Contradictions/Notes: While ONAG provides the algebraic foundation for surreal numbers, modern computational implementations often use more efficient "dyadic" representations rather than the full set-theoretic construction to avoid the complexities of transfinite induction in finite memory.

Last updated: 2026-04-16