$L^p$-improving bounds for spherical maximal operators over restricted dilation sets: radial improvement

In this paper, we study the spherical maximal operator $M_E$ over $E\subset [1,2]$, restricted to radial functions. In higher dimensions $d\geq 3$, we establish a complete range of $L^p-$improving estimates for $M_E$. In two dimensions, sharp results are also obtained for quasi-Assouad regular sets $E$. A notable feature is that the high-dimensional results depend solely on the upper Minkowski dimension, while the two-dimensional results also involve other concepts in fractal geometry such as the Assouad spectrum. Additionally, the geometric shapes of the regions corresponding to the sharp $L^p-$improving bounds differ significantly between the two cases.