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Oscillation of the perturbed Hill equation and the lower spectrum of radially periodic Schrödinger operators in the plane. (English) Zbl 0918.34039
Summary: Generalizing the classical result of Kneser, the author shows that the Sturm-Liouville equation with periodic coefficients and an added perturbation term $-{c}^{2}/{r}^{2}$ is oscillatory or nonoscillatory (for $r\to \infty$) at the infimum of the essential spectrum, depending on whether ${c}^{2}$ surpasses or stays below a critical threshold. An explicit characterization of this threshold value is given. Then this oscillation criterion is applied to the spectral analysis of two-dimensional rotation symmetric Schrödinger operators with radially periodic potentials, revealing the surprising fact that (except in the trivial case of a constant potential) these operators always have infinitely many eigenvalues below the essential spectrum.
##### MSC:
 34C10 Qualitative theory of oscillations of ODE: zeros, disconjugacy and comparison theory 34L05 General spectral theory for OD operators 35P15 Estimation of eigenvalues and upper and lower bounds for PD operators 34D15 Singular perturbations of ODE 34L40 Particular ordinary differential operators