Proceedings of the London Mathematical Society Advance Access originally published online on March 12, 2008
Proceedings of the London Mathematical Society 2008 97(1):97-116; doi:10.1112/plms/pdm058
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© 2008 London Mathematical Society
On time regularity and related conditions for power-bounded operators
Department of Mathematics
Macquarie University
NSW 2109
Australia
Received 2 April 2007. Revision received 14 November 2007.
Let T be a bounded linear operator in a complex Banach space. Our main result gives various characterizations of the condition: T is power-bounded and an estimate ||(I – T)Tn ||
cn– 1/2 holds for all positive integers n. In particular, this condition holds if and only if T = β S + (1 – β)I, for some β
(0, 1) and some power-bounded operator S; or if and only if T is power-bounded and the discrete semigroup (Tn) is dominated by the continuous semigroup (e– t(I – T))t
0 in a natural sense. As a consequence of our main results, for 1/2 <
1 we characterize the condition that T is power-bounded and ||(I – T)Tn ||
c n–
for all n, in terms of estimates on the semigroup e–t(I – T).
2000 Mathematics Subject Classification 47A30, 47A10 (primary), 47D06 (secondary).