Definition 15.1.1 (Measurable Function). Let $(X, \cm)$ and $(Y, \cn)$ be measurable spaces and $f: X \to Y$ be a mapping, then $f$ is $(\cm, \cn)$-measurable if $f^{-1}(E) \in \cm$ for all $E \in \cn$.
Definition 15.1.1 (Measurable Function). Let $(X, \cm)$ and $(Y, \cn)$ be measurable spaces and $f: X \to Y$ be a mapping, then $f$ is $(\cm, \cn)$-measurable if $f^{-1}(E) \in \cm$ for all $E \in \cn$.