Maximal avoidance conjecture for the patterns M2m1M_{2m-1} and M2mM_{2m}

From papers

Let Sn(q)S_n(q) denote the number of permutations of length nn that avoid a pattern qq. For m2m\geq 2, define M2mM_{2m} to be the pattern 132(2m1)(2m2)2m132\cdots(2m-1)(2m-2)2m, and define M2m1M_{2m-1} to be the pattern obtained from M2mM_{2m} by removing its first entry and relabeling; thus M2m1=21(2m2)(2m3)(2m1)M_{2m-1}=21\cdots(2m-2)(2m-3)(2m-1). Maximal-MM conjecture. For every m2m\geq 2, both of the following hold: (A) for every positive integer nn and every pattern qq of length 2m12m-1,

Sn(q)Sn(M2m1);S_n(q)\leq S_n(M_{2m-1});

and (B) for every positive integer nn and every pattern qq of length 2m2m,

Sn(q)Sn(M2m).S_n(q)\leq S_n(M_{2m}).

This is a stronger, parity-specific version supported by numerical evidence of the conjecture that layered patterns maximize the number of avoiding permutations. Its general validity remains open.

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Sources & referencesView supporting material

Primary source

Miklos Bona, “On the Best Upper Bound for Permutations Avoiding A Pattern of a Given Length”, arXiv:1209.2404 (2012).

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