Gradient regularity and potential estimates for fractional drift--diffusion equations in the critical and subcritical ranges

Abstract: We establish scale-invariant interior $C<sup>{1,α}$ estimates for bounded viscosity solutions of $(-Δ)^{su+b\cdot\nabla} u=f$ for $s\in[1/2,1)$ with locally Hölder $b$ and $f$. The critical case uses Silvestre's parabolic theorem; the subcritical case uses Schauder estimates and interpolation. Applying this viscosity estimate to drifted Green sections, for finite Radon data above the critical order we obtain sharp solution and gradient potentials of orders $2s$ and $2s-1$, together with weak-*--to--strong local $W<sup>{1,1}$ stability; hence the Green-potential SOLA is approximation-independent. For the normalized whole-space kernels, we identify the classical second-order limits as $s\uparrow1$, including the logarithmic kernel in dimension two; at the critical order, the whole-space gradient becomes a zero-order singular integral. For zero-exterior problems with compactly supported drift, we also prove $$u/d^s\in C<sup>{s-\varepsilon}(\overlineΩ)$$ and identify the obstruction when the drift reaches the boundary.