If U is a suitably continuous
representation of a locally compact abelian group G by means of isometries on a
Banach space X, μ → U(μ) its extension to a representation of the convolution
algebra M(G) and sp(U) the spectrum of U, then the spectrum of U(μ) is not always
equal to μ(sp(U))−, but it is so if the continuous part of μ is absolutely
continuous.