Some classes of smooth bimodules over II${}_1$ factors and their associated 1-cohomology spaces

Abstract

We study several classes of Banach bimodules over a II${}_1$ factor $M$, endowed with topologies that make them "smooth" with respect to $L^p$-norms implemented by the trace on $M$. Letting $M\subset \B= \B(L^2M)$, and $2\le p<\infty$, we consider: $(1)$ the space $\B(p)$, obtained as the completion of $\B$ in the norm \[ \vertiii{T}_p := \sup \{|φ(T)| \mid φ\in \B^*, \sup\{|φ(xYz)| \mid Y\in (\B)_1, x, z \in M\cap (L^pM)_1\} \leq 1 \}; \] $(2)$ the subspace $\K(p)\subset \B(p)$, obtained as the closure in $\B(p)$ of the space of compact operators $\K(L^2M)$; $(3)$ the space $\K_p\subset \B$ of operators that are $\vertiii{ \, \cdot \, }_p$-limits of bounded sequences of operators in $\K(L^2M)$. We prove that $\K_p$ are all equal to the {\it $\tau$-rank-completion} of $\K(L^2M)$ in $\B$, defined by \begin{align} \text{\rm q}\K_M:= \{K\in \B(L^2M) \mid & \exists K_n \in \K(L^2M), p_n\in \mathcal P(M), \nonumber \\ & \lim_n \|p_n(K-K_n)p_n\|= 0, \lim_nτ(1-p_n)=0\}. \nonumber \end{align} We show that any separable II${}_1$ factor $M$ admits non-inner derivations into $\text{\rm q}\K_M$, but that any derivation $δ:M \rightarrow \text{\rm q}\K_M$ is a pointwise limit in $\tau$-rank-metric of inner derivations.

final version, published in JFA in 2024