Large Perturbations of Nest Algebras

Abstract

Let $\mathcal{M}$ and $\mathcal{N}$ be nests on separable Hilbert space. If the two nest algebras are distance less than 1 ($d(\mathcal{T}(\mathcal{M}),\mathcal{T}(\mathcal{N}))<1$), then the nests are distance less than 1 ($d(\mathcal{M},\mathcal{N})<1$). If the nests are distance less than 1 apart, then the nest algebras are similar, i.e. there is an invertible $S$ such that $S\mathcal{M}=\mathcal{N}$, so that $S\mathcal{T}(\mathcal{M})S^{-1}=\mathcal{T}(\mathcal{N})$. However there are examples of nests closer than 1 for which the nest algebras are distance 1 apart.

Minor changes including a correction in the proof of Theorem 2.2. To appear in Integral Equations & Operator Theory