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Homomesy in products of two chains. (English) Zbl 1319.05151
Summary: Many invertible actions $$\tau$$ on a set $$\mathcal{S}$$ of combinatorial objects, along with a natural statistic $$f$$ on $$\mathcal{S}$$, exhibit the following property which we dub homomesy: the average of $$f$$ over each $$\tau$$-orbit in $$\mathcal{S}$$ is the same as the average of $$f$$ over the whole set $$\mathcal{S}$$. This phenomenon was first noticed by D. I. Panyushev [Eur. J. Comb. 30, No. 2, 586–594 (2009; Zbl 1165.06001)] in the context of the rowmotion action on the set of antichains of a root poset; D. Armstrong et al. [Trans. Am. Math. Soc. 365, No. 8, 4121–4151 (2013; Zbl 1271.05011)] proved Panyushev’s conjecture. We describe a theoretical framework for results of this kind that applies more broadly, giving examples in a variety of contexts. These include linear actions on vector spaces, sandpile dynamics, Suter’s action on certain subposets of Young’s Lattice, Lyness 5-cycles, promotion of rectangular semi-standard Young tableaux, and the rowmotion and promotion actions on certain posets. We give a detailed description of the latter situation for products of two chains.

##### MSC:
 05E18 Group actions on combinatorial structures 05E10 Combinatorial aspects of representation theory 06A11 Algebraic aspects of posets
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