G. Citti et G. Difazio, HOLDER-CONTINUITY OF THE SOLUTIONS FOR OPERATORS WHICH ARE A SUM OF SQUARES OF VECTOR-FIELDS PLUS A POTENTIAL, Proceedings of the American Mathematical Society, 122(3), 1994, pp. 741-750
In this paper we study the local Holder-regularity of weak solutions t
o Lu + Vu = 0 where L is a Hormander hypoelliptic operator and the pot
ential V belongs to a new class of functions which is the natural exte
nsion of Morrey spaces to this situation. We improve a recent result o
f Citti, Garofalo, and Lanconelli.