Optimal elliptic regularity: a comparison between local and nonlocal equations

Given $L\ge 1$, we discuss the problem of determining the highest $\alpha =\alpha (L)$ such that any solution to a homogeneous elliptic equation in divergence form with ellipticity ratio bounded by $L$ is in $C^\alpha_{\rm loc}$. This problem can be formulated both in the classical and non-local framework. In the classical case it is known that $\alpha(L)\gtrsim {\rm exp}(-CL^\beta)$,forsome$C, \beta\geq 1$dependingonthedimension$N\geq 3$\mathrm{.}Weshowthatinthenon-localcase,$\alpha(L)\gtrsim L^{-1-\delta}$ for all $\delta >0$.