Global Schauder estimates for nondivergence stationary operators modeled on homogeneous Hörmander vector fields

In this paper we prove global regularity results and Schauder estimates for non-divergence stationary operators of the form L=\sum_{i,j=1}^m a_{ij}(x) X_i X_j, where X_1, …, X_m are homogeneous (but not necessarily left-invariant) Hörmander vector fields in R^n (n>m), and [a_{ij}(x)] is a symmetric uniformly positive-definite matrix with Hölder-continuous entries w.r.t. the control distance induced by the vector fields X_1, …, X_m.