P. Bougerol et L. Elie, EXISTENCE OF POSITIVE HARMONIC-FUNCTIONS ON GROUPS AND ON COVERING MANIFOLDS, Annales de l'I.H.P. Probabilites et statistiques, 31(1), 1995, pp. 59-80
It is shown, in particular, that a centered probability measure on a l
inear group of exponential growth has non-constant positive harmonic f
unctions. The same conclusion holds also for the Laplace-Beltrami oper
ator on a co-compact Riemannian covering, with such a deck group.