D. Danielli, REGULARITY AT THE BOUNDARY FOR SOLUTIONS OF NONLINEAR SUBELLIPTIC EQUATIONS, Indiana University mathematics journal, 44(1), 1995, pp. 269-286
In this article we establish an estimate, in terms of subelliptic p-ca
pacity, for the modulus of continuity at the boundary of the solution
to the Dirichlet problem associated to a class of subelliptic quasilin
ear equations. We infer from it the sufficiency part of a Wiener type
criterion for the regularity of boundary points.