A non-smooth continuous unitary representation of a Banach-Lie group

Daniel Beltita, Karl-Hermann Neeb

Published 2008-11-26Version 1

In this note we show that the representation of the additive group of the Hilbert space $L^2([0,1],\R)$ on $L^2([0,1],\C)$ given by the multiplication operators $\pi(f) := e^{if}$ is continuous but its space of smooth vectors is trivial. This example shows that a continuous unitary representation of an infinite dimensional Lie group need not be smooth.