The generating-function conjecture for valid hook configurations avoiding length-3 and length-4 patterns

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Let Av⁡n(σ1,…,σr)\operatorname{Av}_n(\sigma_1,\ldots,\sigma_r) denote the permutations of length nn avoiding the listed patterns, and let VHC⁡(S)\operatorname{\mathsf{VHC}}(\mathcal{S}) denote the set of valid hook configurations associated with a permutation class S\mathcal{S}. Then the generating-function conjecture.

∑n≥0∣VHC⁡(Av⁡n(132,3241))∣xn=∑n≥0∣VHC⁡(Av⁡n(231,2143))∣xn=1+x2−1−4x+2x2+x42x.\sum_{n\geq 0}|\operatorname{\mathsf{VHC}}(\operatorname{Av}_n(132,3241))|x^n=\sum_{n\geq 0}|\operatorname{\mathsf{VHC}}(\operatorname{Av}_n(231,2143))|x^n=\frac{1+x^2-\sqrt{1-4x+2x^2+x^4}}{2x}.

This conjecture proposes equal generating functions for valid hook configurations in two permutation-avoidance classes defined by one length-33 and one length-44 pattern. The paper presents it as a direction for future enumeration, and no resolution is supplied here.

References

Primary source

Colin Defant, “Motzkin Intervals and Valid Hook Configurations”, arXiv:1904.10451 (2023).

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