“…More precisely, as a consequence of his more general result, it can be stated, that the equality in (1.2) is attained iff the Riemannian manifold (M, g) is isometrical to the unit sphere S n (1) endowed with the round metric, as (1.1) holds. This result has provoked a similar question in the sub-Riemannian geometry and in particular in the CR geometry, where the problem is successfully solved, see [28,29,35].…”