“…For locally definitizable operators in the context of (indefinite) Sturm-Liouville problems we refer to [15,20,27,64], for λ-dependent boundary value problems see [16,19,63] in the context of PT -symmetric operators see [10,11], in the study of partial differential equations [17,29], for a special form of the Krein-Naimark formula see [13,23] and in the study of problems of Klein-Gordon type see [56,58]. [67] and to the monographs [3,35].…”