On oscillatory elliptic equations on manifolds
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 by A. Baider and E. A. Feldman PDF
 Trans. Amer. Math. Soc. 258 (1980), 495504 Request permission
Abstract:
In this note we investigate the possibility of oscillatory behavior for a secondorder selfadjoint elliptic operators on noncompact Riemannian manifolds (M, g). Let A be such an operator which is semibounded below and symmetric on $C_0^\infty (M) \subseteq {L^2}(M, d\mu )$ where $d\mu$ is a volume element on M. If $\varphi$ is a ${C^\infty }$ function such that $A\varphi = \lambda \varphi$, we would naively say that $\varphi$ is oscillatory (and by extension $\lambda$ is oscillatory if it possesses such an eigenfunction $\varphi$) if $M  {\varphi ^{  1}}(0)$ has an infinite number of bounded connected components. For technical reasons this is not quite adequate for a definition. However, in §1 we give the usual definition of oscillatory which is a slight generalization of the one above. Let ${\Lambda _0}$ be the number below which this phenomenon cannot occur; ${\Lambda _0}$ is the oscillatory constant for the operator A. In that A is semibounded and symmetric on $C_0^\infty (M) \subseteq {L^2}(M, d\mu )$, A has a Friedrichs extension. Let ${\Lambda _c}$ be the bottom of the continuous spectrum of the Friedrichs extension of A. Our main result is ${\Lambda _0} = {\Lambda _c}$.References

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Additional Information
 © Copyright 1980 American Mathematical Society
 Journal: Trans. Amer. Math. Soc. 258 (1980), 495504
 MSC: Primary 58G25; Secondary 35B05, 35J15
 DOI: https://doi.org/10.1090/S00029947198005581866
 MathSciNet review: 558186