p-Schatten commutators of projections

Abstract

Let $H=H_+\oplus H_-$ be a fixed orthogonal decomposition of the complex Hilbert space $H$ in two infinite dimensional subspaces. We study the geometry of the set $P^p$ of selfadjoint projections in the Banach algebra $$ {\cal A}^p=\{A\in B(H): [A,E_+]\in B_p(H)\}, $$ where $E_+$ is the projection onto $H_+$ and $B_p(H)$ is the Schatten ideal of $p$-summable operators ($1\le p <\infty$). The norm in ${\cal A}^p$ is defined in terms of the norms of the matrix entries of the operators given by the above decomposition. The space $P^p$ is shown to be a differentiable $C^\infty$ submanifold of ${\cal A}^p$, and a homogeneous space of the group of unitary operators in ${\cal A}^p$. The connected components of $P^p$ are characterized, by means of a partition of $P^p$ in nine classes, four discrete classes and five essential classes: - the first two corresponding to finite rank or co-rank, with the connected components parametrized by theses ranks; - the next two discrete classes carrying a Fredholm index, which parametrizes its components; - the remaining essential classes, which are connected.