The Multiset Partition Algebra

We introduce the multiset partition algebra $\mathcal{MP}_k(ξ)$ over $F[ξ]$, where $F$ is a field of characteristic $0$ and $k$ is a positive integer. When $ξ$ is specialized to a positive integer $n$, we establish the Schur-Weyl duality between the actions of resulting algebra $\mathcal{MP}_k(n)$ and the symmetric group $S_n$ on $\text{Sym}^k(F^n)$. The construction of $\mathcal{MP}_k(ξ)$ generalizes to any vector $λ$ of non-negative integers yielding the algebra $\mathcal{MP}_λ(ξ)$ over $F[ξ]$ so that there is Schur-Weyl duality between the actions of $\mathcal{MP}_λ(n)$ and $S_n$ on $\text{Sym}^λ(F^n)$. We find the generating function for the multiplicity of each irreducible representation of $S_n$ in $\text{Sym}^λ(F^n)$, as $λ$ varies, in terms of a plethysm of Schur functions. As consequences we obtain an indexing set for the irreducible representations of $\mathcal{MP}_k(n)$, and the generating function for the multiplicity of an irreducible polynomial representation of $GL_n(F)$ when restricted to $S_n$. We show that $\mathcal{MP}_λ(ξ)$ embeds inside the partition algebra $\mathcal{P}_{|λ|}(ξ)$. Using this embedding, over $F$, we prove that $\mathcal{MP}_λ(ξ)$ is a cellular algebra, and $\mathcal{MP}_λ(ξ)$ is semisimple when $ξ$ is not an integer or $ξ$ is an integer such that $ξ\geq 2|λ|-1$. We give an insertion algorithm based on Robinson-Schensted-Knuth correspondence realizing the decomposition of $\mathcal{MP}_λ(n)$ as $\mathcal{MP}_λ(n)\times \mathcal{MP}_λ(n)$-module.

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