Multi-input control-affine systems static feedback equivalent to a   triangular form and their flatness
  In this paper, we give a complete geometric characterization of control systems, with m+1 inputs, locally static feedback equivalent to a triangular form compatible with the chained form, for m=1, respectively with the m-chained form, for m>1. They are x-flat systems. We provide a system of first order PDE's to be solved in order to find all x-flat outputs, for m=1, respectively all minimal x-flat outputs, for m>1. We illustrate our results by examples, in particular by an application to a mechanical system: the coin rolling without slipping on a moving table. 
Shunjie Li, Florentina Nicolau, Witold Respondek
math.OC