Improved ${\mathrm{\ell }}^p$-Boundedness for Integral $k$-Spherical Maximal Functions

We improve the range of ${\mathrm{\ell }}^p({\mathbb{Z}}^d)$-boundedness of the integral $k$-spherical maximal functions introduced by Magyar. The previously best known bounds for the full $k$-spherical maximal function require the dimension $d$ to grow at least cubicly with the degree $k$. Combining ideas from our prior work with recent advances in the theory of Weyl sums by Bourgain, Demeter, and Guth and by Wooley, we reduce this cubic bound to a quadratic one. As an application, we deduce improved bounds in the ergodic Waring–Goldbach problem.