Some Zaremba-Hopf-Oleinik boundary comparison principles at characteristic points.

In this talk I will discuss the validity of the Hopf lemma for certain degenerate-elliptic equations at characteristic boundary points. In the literature there are some positive results under the assumption that the boundary of the domain reflects the underlying geometry of the specific operator. I will mainly focus on conditions on the boundary which are suitable for some families of degenerate operators, also in presence of first order terms. This is a joint work with Vittorio Martino.