On Liouville-type theorems and the uniqueness of the positive Cauchy   problem for a class of hypoelliptic operators
  This note contains a representation formula for positive solutions of linear degenerate second-order equations of the form $$ \partial_t u (x,t) = \sum_{j=1}^m X_j^2 u(x,t) + X_0 u(x,t) \qquad (x,t) \in \mathbb{R}^N \times\, ]- \infty ,T[,$$ proved by a functional analytic approach based on Choquet theory. As a consequence, we obtain Liouville-type theorems and uniqueness results for the positive Cauchy problem. 
Alessia E. Kogoj, Y. Pinchover and S. Polidoro
math.FA math.AP