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Hodge structures on posets
Authors:Phil Hanlon
Affiliation:Department of Mathematics, University of Michigan, Ann Arbor, Michigan 48109-1003
Abstract:Let $ P$ be a poset with unique minimal and maximal elements $ hat{0}$ and $ hat{1}$. For each $ r$, let $ C_r(P)$ be the vector space spanned by $ r$-chains from $ hat{0}$ to $ hat{1}$ in $ P$. We define the notion of a Hodge structure on $ P$ which consists of a local action of $ S_{r+1}$ on $ C_r$, for each $ r$, such that the boundary map $ partial_r: C_rto C_{r-1}$ intertwines the actions of $ S_{r+1}$ and $ S_r$ according to a certain condition.

We show that if $ P$ has a Hodge structure, then the families of Eulerian idempotents intertwine the boundary map, and so we get a splitting of $ H_r(P)$ into $ r$ Hodge pieces.

We consider the case where $ P$ is $ mathcal{B}_{n,k}$, the poset of subsets of $ {1,2,dots, n}$ with cardinality divisible by $ k$ $ (k$ is fixed, and $ n$ is a multiple of $ k)$. We prove a remarkable formula which relates the characters $ mathcal{B}_{n,k}$ of $ S_n$ acting on the Hodge pieces of the homologies of the $ mathcal{B}_{n,k}$ to the characters of $ S_n$ acting on the homologies of the posets of partitions with every block size divisible by $ k$.

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