Symmetrization for Linear and Nonlinear Fractional Parabolic Equations of Porous Medium Type

We establish symmetrization results for the solutions of the linear fractional diffusion equation $\partial_t u +(-\Delta)^{\sigma/2}u=f$ and its elliptic counterpart $h v +(-\Delta)^{\sigma/2}v=f$, $h>0$, using the concept of comparison of concentrations. The results extend to the nonlinear version, $\partial_t u +(-\Delta)^{\sigma/2}A(u)=f$, but only when the nondecreasing function$A:\re_+\to\re_+$ is concave. In the elliptic case, complete symmetrization results are proved for $\,B(v) +(-\Delta)^{\sigma/2}v=f$ \ when $B(v)$ is a convex nonnegative function for $v>0$ with $B(0)=0$, and partial results hold when $B$ is concave. Remarkable counterexamples are constructed for the parabolic equation when $A$ is convex, resp. for the elliptic equation when $B$ is concave. Such counterexamples do not exist in the standard diffusion case $\sigma=2$.