Unimodality of partitions with distinct parts inside Ferrers shapes

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We investigate the rank-generating function Fλ of the poset of partitions contained inside a given shifted Ferrers shape λ. When λ has four parts, we show that Fλ is unimodal when λ=⟨n, n−1, n−2, n−3⟩, for any n≥4, and that unimodality fails for the doubly-indexed, infinite family of partitions of the form λ=⟨n, n−t, n−2t, n−3t⟩, for any given t≥2 and n large enough with respect to t. When λ has b≤3 parts, we show that our rank-generating functions Fλ are all unimodal. However, the situation remains mostly obscure for b≥5. In general, the type of results that we obtain present some remarkable similarities with those of the 1990 paper of D. Stanton, who considered the case of partitions inside ordinary (straight) Ferrers shapes.

Along the way, we also determine some interesting q-analogs of the binomial coefficients, which in certain instances we conjecture to be unimodal. We state several other conjectures throughout this note, in the hopes to stimulate further work in this area. In particular, one of these will attempt to place into a much broader context the unimodality of the posets M(n) of staircase partitions, for which determining a combinatorial proof remains an outstanding open problem.

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© 2015 Elsevier Ltd. All rights reserved. Publisher’s version of record: https://doi.org/10.1016/j.ejc.2015.03.007

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European Journal of Combinatorics