Extension and averaging operators for finite fields

Abstract

In this paper we study $L^p-L^r$ estimates of both extension operators and averaging operators associated with the algebraic variety $S=\{x\in {\mathbb F}_q^d: Q(x)=0\}$ where $Q(x)$ is a nondegenerate quadratic form over the finite field ${\mathbb F}_q.$ In the case when $d\geq 3$ is odd and the surface $S$ contains a $(d-1)/2$-dimensional subspace, we obtain the exponent $r$ where the $L^2-L^r$ extension estimate is sharp. In particular, we give the complete solution to the extension problems related to specific surfaces $S$ in three dimension. In even dimensions $d\geq 2$, we also investigates the sharp $L^2-L^r$ extension estimate. Such results are of the generalized version and extension to higher dimensions for the conical extension problems which Mochenhaupt and Tao studied in three dimensions. The boundedness of averaging operators over the surface $S$ is also studied. In odd dimensions $d\geq 3$ we completely solve the problems for $L^p-L^r$ estimates of averaging operators related to the surface $S.$ On the other hand, in the case when $d\geq 2$ is even and $S$ contains a $d/2$-dimensional subspace, using our optimal $L^2-L^r$ results for extension theorems we, except for endpoints, have the sharp $L^p-L^r$ estimates of the averaging operator over the surface $S$ in even dimensions.