Abstract. We obtain L p eigenfunction bounds for the harmonic oscillator H = −∆+x 2 in R n and for other related operators, improving earlier results of Thangavelu and Karadzhov. We also construct suitable counterexamples which show that our estimates are sharp.
IntroductionThe question of obtaining L p eigenfunction bounds for elliptic operators on compact manifolds has been considered in Sogge's work, for which we refer the reader to his book [11]. The L p eigenfunction bounds in [11] are sharp, and turn out to be related to a variable coefficient version of the restriction theorem, and further to a phase curvature condition for Fourier integral operators. In this analysis a special role is played by the Laplace-Beltrami operator for the sphere, which is the worst case because it has many highly concentrated eigenfunctions. This is connected to the fact that it has a periodic Hamilton flow.In this article we consider the problem of obtaining L p eigenfunction bounds for the Hermite operator H = −∆ + x 2 in R n and also for a larger class of related operators of the form H V = −∆ + V . Within this class the Hermite operator plays a role similar to that of the spherical Laplacian, in that it has a periodic Hamilton flow and many highly concentrated eigenfunctions.This question has received considerable interest in the context of Riesz summability for the harmonic oscillator in the work of Thangavelu [12], [14], [13] and Karadzhov [7].Our interest in it has a different source, namely the strong unique continuation problem for parabolic equations. In this context the work of Thangavelu and Karadzhov has already found applications in Escauriaza [2] and EscauriazaVega [3]. Further applications are contained in a forthcoming paper of the authors. We note in passing that the related strong unique continuation problem for second order elliptic operators is related to the eigenfunction bounds for the spherical harmonics. This was first observed in work of Jerison [6]; see also the authors paper [10] and further references therein.2000 Mathematics Subject Classification. 35S05, 35B60.