Uniform Schauder estimates for regularized hypoelliptic equations

In this paper we are concerned with a family of elliptic operators represented as sum of square vector fields and the limit operator is hypoelliptic. Here we establish Schauder's estimates, uniform with respect to the parameter , of solution of the approximated equation, using a modification of the lifting technique of Rothschild and Stein. These estimates can be used in particular while studying regularity of viscosity solutions of nonlinear equations represented in terms of vector fields.