This paper presents a dimension-free Harnack type inequality for heat
semigroups on manifolds, from which a dimension-free lower bound is ob
tained for the logarithmic Sobolev constant on compact manifolds and a
new criterion is proved for the logarithmic Sobolev inequalities (abb
rev. LSI) on noncompact manifolds. As a result, it is shown that LSI m
ay hold even though the curvature of the operator is negative everywhe
re.