“…In this paper we consider the eigenvalue problem associated with the singular indefinite Sturm-Liouville differential expression − (py ) + qy = λwy in L 2 |w| (a, b), (1.1) where −∞ a < b ∞, the functions p, q, w are real-valued and w changes sign on (a, b). Such a problem is called indefinite and the indefinite nature, that nonreal spectral points may appear, was noticed by Haupt [11], Richardson [19] at the beginning of the last century and has attracted a lot of attention in the recent years, see [1,2,[8][9][10]14]. For a review of early works on indefinite problems, see [15].…”