M. Mbekhta et J. Zemanek, ON THE UNIFORM ERGODIC THEOREM AND THE SP ECTRUM, Comptes rendus de l'Academie des sciences. Serie 1, Mathematique, 317(12), 1993, pp. 1155-1158
The sequence of the arithmetic means of the powers of an operator T on
a Banach space X converges in the operator norm if and only if n(-1)T
(n) converges to zero as n --> + infinity and the range (I-T)(m)X is c
losed for a certain m=1, 2,.... For the Riesz operators, other equival
ent conditions are obtained. Also a theorem of Gelfand-Hille type is g
iven in terms of the operator means.