H\"older regularity and gradient estimates H\"older regularity and gradient estimates for SDEs driven by cylindrical $\alpha$-stable processes

We establish H\"older regularity and gradient estimates for the transition semigroup of the solutions to the following SDE: $${\rm d} X_t=\sigma (t, X_{t-}){\rm d} Z_t+b (t, X_t){\rm d} t,\ \ X_0=x\in{\mathbb R}^d,$$ where $( Z_t)_{t\geq 0}$ is a $d$-dimensional cylindrical $\alpha$-stable process with $\alpha \in (0, 2)$, $\sigma (t, x):{\mathbb R}_+\times{\mathbb R}^d\to{\mathbb R}^d\otimes{\mathbb R}^d$ is bounded measurable, uniformly nondegenerate and Lipschitz continuous in $x$ uniformly in $t$, and $b (t, x):{\mathbb R}_+\times{\mathbb R}^d\to{\mathbb R}^d$ is bounded $\beta$-H\"older continuous in $x$ uniformly in $t$ with $\beta\in[0,1]$ satisfying $\alpha+\beta>1$. Moreover, we also show the existence and regularity of the distributional density of $X (t, x)$. Our proof is based on Littlewood-Paley's theory.