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Published online by Cambridge University Press: 28 December 2015
We characterize Koopman one-parameter
$C_{0}$ -groups, in the class of all unitary one-parameter
$C_{0}$ -groups on
$L_{2}(X)$ , as those that preserve
$L_{\infty }(X)$ and for which the infinitesimal generator is a derivation on the bounded functions in its domain.