Conjecture on enumerating permutations avoiding classical and arrow patterns

From papers

Let an(σ,τ)a_n(\sigma,\tau) denote the number of permutations in the relevant class avoiding the classical pattern σ\sigma and arrow pattern τ\tau. For n2n\geq 2, let FnF_n be the nn-th Fibonacci number and MnM_n the nn-th Motzkin number.

Enumeration conjecture.

an(123,(12;13))=2nn,a_n(123,(12;1\to3))=2^n-n, an(321,(12;13))=F2n1,a_n(321,(12;1\to3))=F_{2n-1},

and

an(321,(12;12))=Mn.a_n(321,(12;1\to2))=M_n.

These conjectured formulas concern cases combining classical pattern avoidance with arrow pattern avoidance. The surrounding discussion identifies these as examples of sequences that appear in unexplored cases; the paper does not provide a proof or resolution.

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Sources & referencesView supporting material

Primary source

Kassie Archer and Robert P. Laudone, “Arrow pattern avoidance in permutations: structure and enumeration”, arXiv:2603.04218 (2026).

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