The trace problem for vector fields satisfying H\"ormander's condition
  Trace theorems are proved for non-isotropic Sobolev and $L^p$-Lipschitz spaces defined by vector fields satisfying H\"ormander's bracket condition of order 2. It is shown that the loss of regularity by traces is the same as in the classical case. 
S. Berhanu and I. Pesenson
math.FA