Estimates near the origin for functional calculus on analytic semigroups

This paper provides sharp lower estimates near the origin for the functional calculus F (−uA) of a generator A of an operator semigroup defined on a sector; here F is given as the Fourier–Borel transform of an analytic functional. The results are linked to the existence of an identity element 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.