Archer et al.'s enumeration conjectures for cyclic permutation pattern avoidance

From papers

Let π\pi be a cyclic permutation in SnS_n, written in one-line notation as π1π2πn\pi_1\pi_2\cdots\pi_n and in cycle form as (c1,c2,,cn)(c_1,c_2,\ldots,c_n). The standard cycle form is the cyclic rotation beginning with 11. Let An(σ1,,σk;τ)\mathcal{A}_n(\sigma_1,\ldots,\sigma_k;\tau) be the set of cyclic permutations in SnS_n that avoid each pattern in {σ1,,σk}\{\sigma_1,\ldots,\sigma_k\} in one-line form and avoid τ\tau in standard cycle form, and write

an(σ1,,σk;τ)=An(σ1,,σk;τ).a_n(\sigma_1,\ldots,\sigma_k;\tau)=|\mathcal{A}_n(\sigma_1,\ldots,\sigma_k;\tau)|.

Archer et al.'s conjectures. For the indicated values of nn, the following enumerations hold:

an(3421,4321;213)=F2n3for every integer n2,a_n(3421,4321;213)=F_{2n-3}\quad\text{for every integer }n\geq2,

where FnF_n is the nn-th Fibonacci number, and

an(1324,1423;213)=(n3)+1for every positive integer n3.a_n(1324,1423;213)=\binom{n}{3}+1\quad\text{for every positive integer }n\geq3.

These conjectures concern simultaneous pattern avoidance in the one-line and standard cycle forms of cyclic permutations. The source presents them as proposed further directions for research, and no resolution is supplied here.

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Sources & referencesView supporting material

Primary source

Junyao Pan, “On two conjectures about pattern avoidance of cyclic permutations”, arXiv:2505.02045 (2025).

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