Kreiss bounded and uniformly Kreiss bounded operators

Abstract

If T is a Kreiss bounded operator on a Banach space, then $\Vert T^n\Vert =O(n)$. Forty years ago Shields conjectured that in Hilbert spaces, $\Vert T^n\Vert = O(\sqrt{n})$. A negative answer to this conjecture was given by Spijker, Tracogna and Welfert in 2003. We improve their result and show that this conjecture is not true even for uniformly Kreiss bounded operators. More precisely, for every $\varepsilon >0$ there exists a uniformly Kreiss bounded operator T on a Hilbert space such that $\Vert T^n\Vert \sim (n+1)^{1-\varepsilon }$ for all $n\in \mathbb {N}$. On the other hand, any Kreiss bounded operator on Hilbert spaces satisfies $\Vert T^n\Vert =O(\frac{n}{\sqrt{\log n}})$. We also prove that the residual spectrum of a Kreiss bounded operator on a reflexive Banach space is contained in the open unit disc, extending known results for power bounded operators. As a consequence we obtain examples of mean ergodic Hilbert space operators which are not Kreiss bounded.