“…This question has a distinguished history that goes back to Hermite, Laguerre, Hurwitz and Pólya-Schur, see [9] and references therein. In particular, in [41] Pólya and Schur characterized all diagonal operators with this property, which led to a rich subsequent literature on this subject [6,8,17,18,19,20,21,26,31,35,40,43,52]. However, it was not until very recently that full solutions to this question -and, more generally, to Problems 1-2 for n = 1 and any open circular domain Ω -were obtained in [6].…”