Diverse regularities for nonlocal parabolic De Giorgi classes I

In this paper we establish diverse Harnack inequalities for elements of a specific energy class, called the nonlocal parabolic (p-homogenous) De Giorgi class. This class encompasses the nonlinear parabolic counterpart of the seminal work of M. Cozzi (J. Funct. Anal., 2017) and embodies local weak solutions to the fractional heat equation. More precisely, we first derive the local boundedness and subsequently prove under minimal tail conditions several weak Harnack inequalities, measure theoretical propagation lemmas, and a full Harnack inequality for nonnegative members of the aforementioned class. Finally, we present a full proof of the local Hölder modulus of continuity, thereby establishing a Liouville-type rigidity property. The results are new even for the linear case, thereby showing that the recent achievements of Kassmann and Weidner (Duke Math. J., 2024) are structural properties, valid regardless of any equation. The techniques and ideas presented in this paper will open the door for further extensions to many natural directions.