Let \((X, \mathscr{A}, \mu)\) be a \(\sigma\)-finite measure space and let \(u\) be a further measure. Show that \(u \leqslant \mu\) entails that \(u=f \mu\) for some (a.e. uniquely determined) density function \(f\) such that \(0 \leqslant f \leqslant 1\).