(x, t)+ ib (x, t))∂/∂ x, a, b∈ C∞(T2; R), b≢ 0. We view L as an operator acting on smooth
functions and present conditions for L to have either a closed range or a finite-
codimensional range. Our results involve, besides condition (P) of Nirenberg and Treves,
the behavior of a+ ib near each one-dimensional Sussmann orbit homotopic to the unit
circle. One of the main goals of our work is to provide some clarification about the role …