Effective Prüfer angles and relative oscillation criteria. (English) Zbl 1167.34009

Authors’ abstract: Summary: We present a streamlined approach to relative oscillation criteria based on effective Prüfer angles adapted to the use at the edges of the essential spectrum.
Based on this we provide a new scale of oscillation criteria for general Sturm-Liouville operators which answer the question whether a perturbation inserts a finite or an infinite number of eigenvalues into an essential spectral gap. As a special case we recover and generalize the Gesztesy-Ünal criterion (which works below the spectrum and contains classical criteria by Kneser, Hartman, Hille, and Weber) and the well-known results by Rofe-Beketov including the extensions by Schmidt.


34C10 Oscillation theory, zeros, disconjugacy and comparison theory for ordinary differential equations
34L05 General spectral theory of ordinary differential operators
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