Conjectured enumeration of 132-avoiding permutations fixed by the fundamental bijection

Let tnk(τ)t_n^k(\tau) denote the number of permutations of length nn avoiding the pattern τ\tau that are fixed by the kkth iterate of the fundamental bijection. For ngeq2ngeq2, the conjectured values for tn2(132)t_n^2(132) are

tn2(132)={k3+3k2+2k1n=3k,k3+4k2+4kn=3k+1,k3+5k2+7k+2n=3k+2.t_n^2(132)=\begin{cases} k^3+3k^2+2k-1 & n=3k,\\ k^3+4k^2+4k & n=3k+1,\\ k^3+5k^2+7k+2 & n=3k+2.\end{cases}

For k3k\geq3, the conjectured values of tnk(132)t_n^k(132) are 3n43n-4 when k=3k=3, 2n12n-1 when k=4k=4, n+2n+2 when k=5k=5, and 55 when k6k\geq6.

These formulas concern the remaining enumeration case after the paper establishes eventual vanishing for the pattern 123123; enumeration for Tk(132)\mathcal{T}^k(132) with k2k\geq2 is presented as complicated and unresolved.

Sources & referencesView supporting material

Primary source

Kassie Archer and Robert P. Laudone, “Pattern avoidance and the fundamental bijection”, arXiv:2407.06338 (2024).

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