“…Indeed, even though many authors have addressed Cauchy problems for degenerate hyperbolic equations in two variables, most studies are restricted to equations where the 𝜕 2 𝜕𝑥 2 term vanishes at an initial line 𝑦 = 𝑦 0 (we refer to [15, §2.3], [149, Section 5.4] and references therein). Much less is known for hyperbolic equations whose 𝜕 2 𝜕𝑦 2 term vanishes at the same initial line: it is known that the Cauchy problem is, in general, not well-posed, and the relevance of determining conditions for its well-posedness has long been pointed out [15, §2.4], but as far as we are aware little progress has been made on this problem (for related work see [122]). The application of spectral techniques to hyperbolic Cauchy problems associated with Sturm-Liouville operators is by no means new, see e.g.…”