Abstract. Consider a compact Riemannian manifold with ergodic geodesic flow. Quantum ergodicity is generalized from orthonormal bases of eigenfunctions of the Laplacian to packets of eigenfunctions. It is shown that this more general result is sharp. Namely, there may exist exceptional packets of eigenfunctions which concentrate on a submanifold.
IntroductionLet M be a compact Riemannian manifold. The Laplacian ∆ of M is a second order self-adjoint differential operator. Moreover, there exist orthonormal bases forIn the case when the geodesic flow on the unit sphere bundle of M is ergodic, Colin de Verdière [2] proved a remarkable theorem. Suppose that A is a zero'th order pseudodifferential operator with symbol a. Then there exists a subsequence φ ki , of density one in φ k , so that, for any A,Here dω is the normalized Liouville measure on the unit sphere bundle S 1 M . An important open problem is whether the equality (0.1) holds for the full orthonormal basis. We contribute to this discussion by broadening the perspective. Definition 1.1 introduces the concept of packets of eigenfunctions, which are certain finite linear combinations of eigenfunctions. Our Theorem 1.4 shows that the result of Colin de Verdière generalizes to such packets of eigenfunctions. The rest of the paper is devoted to establishing the sharpness of this more general result. Namely, there may exist exceptional subsequences, of density zero, where (0.1) fails. Theorem 2.2 constructs manifolds with ergodic geodesic flow which have nonpositive curvature and contain flat cylinders. The proof of Theorem 2.2 is an application of the method of Burns and Gerber [1]. In Theorem 3.5, we produce exceptional sequences of eigenfunction packets supported on the manifolds provided by Theorem 2.2. These packets may be localized to arbitrarily small intervals on the spectrum while maintaining a fixed normalization for the average level spacings.