Multiplicative Modular Nim (MuM)

TL;DR

This paper introduces Multiplicative Modular Nim (MuM), a novel variant of Nim using modular multiplication, establishing a complete theory including a Sprague-Grundy analogue, and extending to finite fields for educational purposes.

Contribution

It presents the first systematic analysis of a multiplicative modular Nim variant, including a new game-theoretic value called the mumber and a decomposition via the Chinese Remainder Theorem.

Findings

MuM's mumbers are equivalent to heap-product modulo m.

Disjunctive sums of MuM games combine via modular multiplication.

MuM decomposes into prime-power subgames using the Chinese Remainder Theorem.

Abstract

We introduce Multiplicative Modular Nim (MuM), a variant of Nim in which the traditional nim-sum is replaced by heap-size multiplication modulo m. We establish a complete theory for this game, beginning with a direct, Bouton-style analysis for prime moduli. Our central result is an analogue of the Sprague-Grundy theorem, where we define a game-theoretic value, the mumber, for each position via a multiplicative mex recursion. We prove that these mumbers are equivalent to the heap-product modulo m, and show that for disjunctive sums of games, they combine via modular multiplication in contrast to the XOR-sum of classical nimbers. For composite moduli, we show that MuM decomposes via the Chinese Remainder Theorem into independent subgames corresponding to its prime-power factors. We extend the game to finite fields F(pn), motivated by the pedagogical need to make the algebra of the AES S-box more accessible. We demonstrate that a sound game in this domain requires a Canonical Heap Model to resolve the many-to-one mapping from integer heaps to field elements. To our knowledge, this is the first systematic analysis of a multiplicative modular variant of Nim and its extension into a complete, non-additive combinatorial game algebra.