${\ell }^{p}$-improving for discrete spherical averages
Annales Henri Lebesgue, Volume 3 (2020) , pp. 959-980.

Keywords${L}^{p}$-improving, discrete averages, discrete maximal functions, circle method, Littlewood–Paley theory

### Abstract

We initiate the theory of ${\ell }^{p}$-improving inequalities for arithmetic averages over hypersurfaces and their maximal functions. In particular, we prove ${\ell }^{p}$-improving estimates for the discrete spherical averages and some of their generalizations. As an application of our ${\ell }^{p}$-improving inequalities for the dyadic discrete spherical maximal function, we give a new estimate for the full discrete spherical maximal function in four dimensions. Our proofs are analogous to Littman’s result on Euclidean spherical averages. One key aspect of our proof is a Littlewood–Paley decomposition in both the arithmetic and analytic aspects. In the arithmetic aspect this is a major arc-minor arc decomposition of the circle method.

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