Hölder continuity for sub-elliptic systems under the sub-quadratic controllable growth in Carnot groups Jialin Wang, Dongni Liao, Zefeng Yu (2013) Rendiconti del Seminario Matematico della Università di Padova
Hypoellipticity, fundamental solution and Liouville type theorem for matrix-valued differential operators in Carnot groups Annalisa Baldi, Bruno Franchi, Maria Carla Tesi (2009) Journal of the European Mathematical Society