On the growth of generalized Fourier coefficients of restricted eigenfunctions (2204.01881v1)

Abstract: Let $(M,g)$ be a smooth, compact, Riemannian manifold and ${\phi_h}$ a sequence of $L^2$-normalized Laplace eigenfunctions on $M$. For a smooth submanifold $H\subset M$, we consider the growth of the restricted eigenfunctions $\phi_h|H$ by testing them against a sequence of functions ${\psi_h}$ on $H$ whose wavefront set avoids $S^*H$. That is, we study what we call the generalized Fourier coefficients: $\langle \phi_h,\psi_h\rangle{L^2(H)}$. We give an explicit bound on these coefficients depending on how the defect measures for the two collections of functions $\phi_h$ and $\psi_h$ relate. This allows us to get a little$-o$ improvement whenever the collection of recurrent directions over the wavefront set of $\psi_h$ is small. To obtain our estimates, we utilize geodesic beam techniques.