Prüfer Transformation and Spectral Analysis for a Sturm--Liouville-Type Equation

We study a second-order differential equation involving a quasi-derivative of the form y^ + s(x)y, leading to a non-self-adjoint Sturm--Liouville-type problem with four coefficient functions. To analyze this equation, we develop a generalized Prüfer transformation that expresses solutions in terms of amplitude and phase variables. The resulting system is nonlinear, reflecting the structure of the original operator. Using this framework, we establish comparison theorems, prove the monotonicity of eigenfunction zeros with respect to the spectral parameter, and derive upper and lower bounds for the eigenvalues.