Relative oscillation theory and essential spectra of Sturm–Liouville operators

We develop relative oscillation theory for general Sturm–Liouville differential expressions of the form1r(−ddxpddx+q) and prove perturbation results and invariance of essential spectra in terms of the real coefficients p, q, r. The novelty here is that we also allow perturbations of the weight function r in which case the unperturbed and the perturbed operator act in different Hilbert spaces.