Continuous-time Kreiss resolvent condition on infinite-dimensional spaces

Abstract:

Given the infinitesimal generator $A$ of a $C_0$-semigroup on the Banach space $X$ which satisfies the Kreiss resolvent condition, i.e., there exists an $M>0$ such that $\| (sI-A)^{-1}\| \leq \frac {M}{\mathrm {Re}(s)}$ for all complex $s$ with positive real part, we show that for general Banach spaces this condition does not give any information on the growth of the associated $C_0$-semigroup. For Hilbert spaces the situation is less dramatic. In particular, we show that the semigroup can grow at most like $t$. Furthermore, we show that for every $\gamma \in (0,1)$ there exists an infinitesimal generator satisfying the Kreiss resolvent condition, but whose semigroup grows at least like $t^\gamma$. As a consequence, we find that for ${\mathbb R}^N$ with the standard Euclidian norm the estimate $\|\exp (At)\| \leq M_1 \min (N,t)$ cannot be replaced by a lower power of $N$ or $t$.