Definition:Koopman Operator on Complex L-2 Space

Definition
Let $\struct {X, \BB, \mu, T}$ be a measure-preserving dynamical system.

Let $\map {L^2} \mu$ be the $L^2$ space of $\mu$.

The Koopman operator $U_T : \map {L^2} \mu \to \map {L^2} \mu$ of $T$ is defined by:
 * $\map {U_T} f = f \circ T$

where $f \circ T$ denotes the composition of $T$ and $f$.