THE LI-YAU INEQUALITY AND APPLICATIONS UNDER A CURVATURE-DIMENSION CONDITION

We prove a global Li-Yau inequality for a general Markov semi group under a curvature-dimension condition. This inequality is stronger than all classical Li-Yau type inequalities known to us. On a Riemannian manifold, it is equivalent to a new parabolic Harnack inequality, both in negative and positive curvature, giving new subsequent bounds on the heat kernel of the semigroup. Under positive curvature we moreover reach ultracontractive bounds by a direct and robust method.