Fishburn Permutations Avoiding 2143 and 3124

Eric S. Egge, Pattern-Avoiding Fishburn Permutations and Ascent Sequences, arXiv:2208.01484v1, Section 10:

Conjecture 10.9. For all n ≥ 1, |Fn(2143, 3124)| = |Sn(231, 4123)|.

A permutation p is Fishburn when there are no indices i and j > i + 1 with p_i = p_j + 1 < p_{i+1}; F_n(B) and S_n(B) are the Fishburn permutations and all permutations of length n avoiding the classical patterns in B.

The theorem D5/S3/Combinatorics/Fishburn/FishburnTenNine.result establishes the statement: the two classes are equinumerous for every n ≥ 1; both have the generating function (1 − x)^3/(1 − 4x + 5x^2 − 3x^3).

Pre-registration issue 11829 records the literature screen: of the papers citing arXiv:2208.01484, only Du and Zhang settle conjectures of the paper (Conjectures 10.14 and 10.17), and none of the located later papers treats this one. This is a bounded negative finding.

1. The active sites of a Fishburn permutation avoiding 2143 and 3124 are characterized completely through its prefix, using layered permutations as an auxiliary class.
2. This gives a complete generating tree for the Fishburn class with explicit inverse, and its count.
3. A separate maximum decomposition of the permutations avoiding 231 and 4123 gives the same generating function.

The statement would fail if some active insertion site were misclassified, if a parent produced a child twice or missed one, or if the two counts differed for some length.

Every structural lemma was checked on all permutations of length at most 10; the counts agree through the lengths reported in the pre-registration.

theorem; the statement is a conjecture of Section 10 of arXiv:2208.01484 and is quantified over every n.

The literature screen is limited to the citation list, the arXiv and GitHub searches and the repository checks recorded above.