Large-order reverse-strategy conjecture for the permutation avoidance game

From papers

Let k3k\geq 3 be the pattern length, let SnS_n denote the symmetric group on nn elements, and let sg(Sn,k)\operatorname{sg}(S_n,k) denote the Sprague–Grundy value of the permutation avoidance game PAP on SnS_n with pattern length kk. The reverse strategy is the strategy in which Player II responds by choosing the reverse of Player I's chosen pattern.

Large-order reverse-strategy conjecture. For every k3k\geq 3, there exists n0(k)n_0(k) such that the reverse strategy is a winning strategy for Player II on SnS_n for all nn0(k)n\geq n_0(k). In particular,

sg(Sn,k)=0\operatorname{sg}(S_n,k)=0

for all such nn.

Computations suggest that small values of nn can obstruct the reverse strategy, while the paper proves that it succeeds for k=4k=4 whenever n10n\geq 10. The conjecture asserts that an analogous eventual result holds for every pattern length k3k\geq 3.

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Sources & referencesView supporting material

Primary source

Henning Ulfarsson, “A Permutation Avoidance Game with Reverse Replies and Monotone Traps”, arXiv:2603.16004 (2026).

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