“…In particular, in [49] an intrinsic Harnack estimate is proved for degenerate parabolic equations with degeneracy of two types: p-Laplacean and porous media (and it has been extended to doubly degenerate equations in [50]). The results in those papers, on the one hand, imply that, if u(x 0 , t 0 ) > 0, then u(x 0 , t) > 0 for all t > t 0 and, on the other, have been used in [51,52] to show that the expansion of the positivity set does occur also with lower order terms in the case of p-Laplace type of degenerate equations. The validity of the analogous result for the porous medium equations still seems to be an open problem.…”