Comparison and regularity results for the fractional Laplacian via   symmetrization methods
  In this paper we establish a comparison result through symmetrization for solutions to some boundary value problems involving the fractional Laplacian. This allows to get sharp estimates for the solutions, obtained by comparing them with solutions of suitable radial problems. Furthermore, we use such result to prove a priori estimates for solutions in terms of the data, providing several regularity results which extend the well known ones for the classical Laplacian. 
Giuseppina Di Blasio and Bruno Volzone
math.AP