Infinite-dimensional features of matrices and pseudospectra

Abstract

Given a Hilbert space operator $T$, the level sets of function $Ψ_T(z)=\|(T-z)^{-1}\|^{-1}$ determine the so-called pseudospectra of $T$. We set $Ψ_T$ to be zero on the spectrum of $T$. After giving some elementary properties of $Ψ_T$ (which, as it seems, were not noticed before), we apply them to the study of the approximation. We prove that for any operator $T$, there is a sequence $\{T_n\}$ of finite matrices such that $Ψ_{T_n}(z)$ tends to $Ψ_{T}(z)$ uniformly on $\C$. In this proof, quasitriangular operators play a special role. This is merely an existence result, we do not give a concrete construction of this sequence of matrices. One of our main points is to show how to use infinite-dimensional operator models in order to produce examples and counterexamples in the set of finite matrices of large order. In particular, we get a result, which means, in a sense, that the pseudospectrum of a nilpotent matrix can be anything one can imagine. We also study the norms of the multipliers in the context of Cowen--Douglas class operators. We use these results to show that, to the opposite to the function $Ψ_{S}$, the function $\|\sqrt{S-z}\,\|$ for certain finite matrices $S$ may oscillate arbitrarily fast even far away from the spectrum.