The first positive eigenvalue of the sub-laplacian on cr spheres
  We prove that the first positive eigenvalue, normalized by the volume, of the sub-Laplacian associated with a strictly pseudoconvex pseudo-Hermitian structure $\\theta$ on the CR sphere S 2n+1 $\subset$ C n+1 , achieves its maximum when $\\theta$ is the standard contact form. 
Amine Aribi (LMPT), Ahmad El Soufi (LMPT)
math.DG