  We establish a Talenti-type symmetrization result in the form of mass
concentration (i.e. integral comparison) for very general linear nonlocal
elliptic problems, equipped with homogeneous Dirichlet boundary conditions. In
this framework, the relevant concentration comparison for the classical
fractional Laplacian can be reviewed as a special case of our main result, thus
generalizing the previous results in [21]. Finally, using an implicit time
discretization techniques, similar results are obtained for the solutions of
Cauchy-Dirichlet nonlocal linear parabolic problems.
