Symmetrization of exterior parabolic problems and probabilistic interpretation

We prove a comparison theorem for the averages of the solutions of two exterior parabolic problems, the second being the “symmetrization” of the first one, by using approximation of the Schwarz symmetrization by polarizations, as it was introduced in [4]. This comparison provides an alternative proof, based on PDEs, of the isoperimetric inequality for the Wiener sausage, which was proved in [14].