On the paving size of a subfactor

Abstract

Given an inclusion of II$_1$ factors $N\subset M$ with finite Jones index, $[M:N]<\infty$, we prove that for any $F\subset M$ finite and $\varepsilon >0$, there exists a partition of $1$ with $r\leq \lceil 16\varepsilon^{-2}\rceil$ $\cdot \lceil 4 [M:N]\varepsilon^{-2}\rceil$ projections $p_1, ..., p_r\in N$ such that $\|\sum_{i=1}^r p_ixp_i - E_{N'\cap M}(x)\|\leq \varepsilon \|x-E_{N'\cap M}(x)\|$, $\forall x\in F$ (where $\lceil \beta \rceil$ denotes the least integer $\geq \beta$). We consider a series of related invariants for $N\subset M$, generically called {\it paving size}.