Sub-Riemannian balls in CR Sasakian manifolds
  We prove global estimates for the sub-Riemannian distance of CR Sasakian manifolds with non negative horizontal Webster-Tanaka Ricci curvature. In particular, in this setting, large sub-Riemannian balls are comparable to Riemannian balls. 
Fabrice Baudoin, Michel Bonnefont
math.DG math.MG