# Analysis of Misere Nim?

My friend likes to impress people by playing 3-5-7 which has three piles of counters of sizes 3, 5 and 7. You can remove any number of coins from a single pile, the last player to move loses.

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This is a winning position for the first player, but With a solid understanding of the game tree she wins nearly every time playing second. She says, it reduces to knowing a few winning positions.

Two piles of the same size is second player win, in the jargon of Combinatorial Game Theory. Here is the pile (5,5).

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If first player moves to (5,n) for n > 1, second player can imitate on the other pile, moving to (n,n). However, if first player moves to (5,1), second player moves to 1 and wins.

The other winning positions she remembers is (3,4,1) and (4,5,1). She can win once she recognizes these positions. Eventually (after losing many times) I told her that (n, n+1,1) is a losing position for any n...

If our game were played in normal play convention (player moving last wins), but real life Nim is played as a misere game. Probably the analysis is similar to normal-play Nim with some modification.

Recently there was a theory of Misere quotients where each game has a commutative monoid assoicated with it. What does that monoid looks like here? Is it finitely generated?

• The winning positions in misère Nim are the same for for normal play (pointwise xor of the sizes in binary is nonzero), except when there are only heaps of size 1 left, in which case it’s reversed (i.e., even number of heaps is winning). Aug 1, 2011 at 15:36
• In particular, (3,4,1) is a first player win, whereas (4,5,1) is a second player win, so your friend or you got it wrong. Aug 1, 2011 at 15:48

Let $\oplus$ denote the bitwise xor operation on natural numbers. It is both well-known and easy to verify that a Nim position $(n_1,\dots,n_k)$ is a second player win in misère Nim if and only if some $n_i>1$ and $n_1\oplus\cdots\oplus n_k=0$, or all $n_i\le1$ and $n_1\oplus\cdots\oplus n_k=1$.
If I understand it correctly, this translates to the following structure. Let $A=(\omega,\oplus,0)$ (in other words, $A$ is the direct sum of countably many copies of the two-element abelian group), let $A_0=\{0,1\}$ be its submonoid, and let $B=(\{0,1\},\lor,0)$ be the two-element semilattice. Then the underlying monoid of the misère quotient of Nim is the submonoid $Q=(A\times\{1\})\cup(A_0\times\{0\})$ of the product monoid $A\times B$, the misère quotient itself being $(Q,\{(0,1),(1,0)\})$. If $(n_1,\dots,n_k)$ is a Nim position, its value in $Q$ is $(n_1\oplus\cdots\oplus n_k,u)$, where $u=0$ if $n_i\in\{0,1\}$ for each $i$, and $u=1$ otherwise. $Q$ has (as a commutative monoid) the infinite presentation $\langle \{a_n:n\in\omega\},b\mid a_n+a_n=b+b=0,a_n+b=a_n+a_0\rangle$. Finitely generated submonoids of $Q$ are finite, hence $Q$ itself is not finitely generated.
(Note that the monoid corresponding to normal play Nim is just $A$.)