Boris Shapiro: On the inverse problem in Polya-Schur theory

Abstract:

Given a linear ordinary differential operator \(T\) with polynomial coefficients, we study the class of closed subsets of the complex plane such that \(T\) sends any polynomial (resp. any polynomial of degree exceeding a given positive integer) with all roots in a given subset to a polynomial with all roots in the same subset or to \(0\). We discuss some general properties of such invariant subsets as well as the problem of existence of the minimal under inclusion invariant subset.