“…There exists an enormous body of literature for these kinds of generalized spectral problems and without any possibility of achieving completeness, we refer, for instance, to [2], [12], [55], [68], [72], [73], [74], [75], [98], [116], [131], and the extensive literature cited therein in the context of general boundary value problems. In the context of indefinite Sturm-Liouville-type boundary value problems we mention, for instance, [6], [8], [11], [13], [14], [15], [16], [18], [19], [20], [21], [23], [30], [31], [32], [37], [47], [52], [56], [83], [84], [86], [89], [90], [91], [99], [100], [117], [132], [135,Chs. 5,11,12], and again no attempt at a comprehensive account of the existing literature is possible due to the enormous volume of the latter.…”