Automatic continuity of operator semigroups in the Calkin algebra

Abstract

We study operator semigroups in the Calkin algebra $\mathcal{Q}(\mathcal{H})$, represented as a subalgebra of the algebra of bounded linear operators on a Hilbert space via one of `canonical' Calkin's representations. Using the BDF theory, we associate with any normal $C_0$-semigroup $(q(t))_{t\geq 0}$ in $\mathcal{Q}(\mathcal{H})$ an extension $Γ\in\mathrm{Ext}(Δ)$, where $Δ$ is the inverse limit of certain compact metric spaces defined purely in terms of the spectrum $σ(A)$ of the generator of $(q(t))_{t\geq 0}$. Then we show that, in natural circumstances, if $(q(t))_{t\geq 0}$ is continuous in the strong operator topology, then it is actually uniformly continuous, although there are $C_0$-semigroups in $\mathcal{Q}(\mathcal{H})$ that are not uniformly continuous.

Theorems & Definitions (29)

• Theorem 2.1: see milnor and BDF
• Definition 2.2
• Definition 2.3
• Remark 2.4
• Definition 2.5
• Remark 2.6
• Lemma 2.7
• proof
• Remark 2.8
• Proposition 3.1