On the validity of the Radon–Nikodym theorem

This paper presents a new general formulation of the Radon–Nikodym theorem in the setting of abstract measure theory. We introduce the notion of weak localizability for a measure and show that this property is both necessary and sufficient for the validity of a Radon–Nikodym-type representation under a natural compatibility relation between measures. The proof relies solely on elementary tools, such as Markov’s inequality and the monotone convergence theorem. In addition to establishing the main result, we provide a constructive approach to envelope functions for families of non-negative measurable functions supported on sets of finite measure.