Kleiner's theorem for unitary representations of posets

Abstract

A subspace representation of a poset $\mathcal S=\{s_1,...,s_t\}$ is given by a system $(V;V_1,...,V_t)$ consisting of a vector space $V$ and its subspaces $V_i$ such that $V_i\subseteq V_j$ if $s_i \prec s_j$. For each real-valued vector $χ=(χ_1,...,χ_t)$ with positive components, we define a unitary $χ$-representation of $\mathcal S$ as a system $(U;U_1,...,U_t)$ that consists of a unitary space $U$ and its subspaces $U_i$ such that $U_i\subseteq U_j$ if $s_i\prec s_j$ and satisfies $χ_1 P_1+...+χ_t P_t= \mathbb 1$, in which $P_i$ is the orthogonal projection onto $U_i$. We prove that $\mathcal S$ has a finite number of unitarily nonequivalent indecomposable $χ$-representations for each weight $χ$ if and only if $\mathcal S$ has a finite number of nonequivalent indecomposable subspace representations; that is, if and only if $\mathcal S$ contains any of Kleiner's critical posets.