“…[15,21,22,25,31,34,38,39,41,66,93], KreinFeller operators [46], λ-dependent boundary value problems, see e.g. [24,30,42,62,63,82], operator polynomials [68,69,71,72,74,75,76,87], second order systems [50,89,90] and in the study of problems of Klein-Gordon type [83].…”