An alternative proof of the a priori $\tan\Theta$ Theorem

Abstract

Let $A$ be a self-adjoint operator in a separable Hilbert space. Suppose that the spectrum of $A$ is formed of two isolated components $\sigma_0$ and $\sigma_1$ such that the set $\sigma_0$ lies in a finite gap of the set $\sigma_1$. Assume that $V$ is a bounded additive self-adjoint perturbation of $A$, off-diagonal with respect to the partition ${\rm spec}(A)=\sigma_0 \cup \sigma_1$. It is known that if $\|V\|<\sqrt{2}{\rm dist}(\sigma_0,\sigma_1)$, then the spectrum of the perturbed operator $L=A+V$ consists of two disjoint parts $\omega_0$ and $\omega_1$ which originate from the corresponding initial spectral subsets $\sigma_0$ and $\sigma_1$. Moreover, for the difference of the spectral projections $E_A(\sigma_0)$ and $E_{L}(\omega_0)$ of $A$ and $L$ associated with the spectral sets $\sigma_0$ and $\omega_0$, respectively, the following sharp norm bound holds: $$\|E_A(\sigma_0)-E_{L}(\omega_0)\|\leq\sin\left(\arctan\frac{\|V\|}{{\rm dist}(\sigma_0,\sigma_1)}\right).$$ In the present note, we give a new proof of this bound for $\|V\|<{\rm dist}(\sigma_0,\sigma_1)$.