A host algebra of a (possibly infinite dimensional) Lie group $G$ is a $C^*$-algebra whose representations are in one-to-one correspondence with certain continuous unitary representations $\pi \colon G \to \U(\cH)$. In this paper we present a new approach to host algebras for infinite dimensional Lie groups which is based on smoothing operators, i.e., operators whose range is contained in the space $\cH^\infty$ of smooth vectors. Our first major result is a characterization of smoothing operators $A$ that in particular implies smoothness of the maps $\pi^A \colon G \to B(\cH), g \mapsto \pi(g)A$. The concept of a smoothing operator is particularly powerful for representations $(\pi,\cH)$ which are semibounded, i.e., there exists an element $x_0 \in\g$ for which all operators $i\dd\pi(x)$, $x \in \g$, from the derived representation are uniformly bounded from above in some neighborhood of $x_0$. Our second main result asserts that this implies that $\cH^\infty$ coincides with the space 查看全文>>