Nagy's perturbation of a non-selfadjoint operator and application to a
Gribov operator in Bargmann space
The purpose of the present paper is to formulate some new supplements to
perturbation theory of linear operators  by considering a
non-analytic perturbation involving more than one perturbation
parameter. An application to a Gribov operator in Bargmann space
illustrates the mathematical problem involved in this paper.