Lower estimates near the origin for functional calculus on operator semigroups

This paper provides sharp lower estimates near the origin for the functional calculus F(−uA)F(−uA) of a generator A of an operator semigroup defined on the (strictly) positive real line; here F is given as the Laplace transform of a measure or distribution. The results are linked to the existence of an identity element or an exhaustive sequence of idempotents in the Banach algebra generated by the semigroup. Both the quasinilpotent and non-quasinilpotent cases are considered, and sharp results are proved extending many in the literature.