“…This nonlinear equations' principal part is subelliptic, and we note that in the nonlinear term, only Xφ(t), XY φ(t), where X, Y in 0 T + 0 T , terms appear. Of course if there is no nonlinear term in this equation, the solution must be real analytic (M being real analytic, so real analytic hypo-ellipticity holds) (see [Tar1], [Ko]). In our case, as the non linear term is quite suitable (it doesn't include XT φ(t) term and T T φ(t) term, where X in 0 T + 0 T and T is the missing direction), it is natural to expect the same result as in the elliptic case.…”