Simple eigenvectors of unbounded operators of the type »normal plus compact«

Abstract

The paper deals with operators of the form $A=S+B$, where $B$ is a compact operator in a Hilbert space $H$ and $S$ is an unbounded normal one in $H$, having a compact resolvent. We consider approximations of the eigenvectors of $A$, corresponding to simple eigenvalues by the eigenvectors of the operators $A_{n}=S+B_{n}$ ($n=1,2, \ldots$), where $B_n$ is an $n$-dimensional operator. In addition, we obtain the error estimate of the approximation.