Editing Radon–Nikodym theorem (section)

===Extension to signed or complex measures===
A similar theorem can be proven for [[Signed measure|signed]] and [[complex measure]]s: namely, that if <math>\mu</math> is a nonnegative σ-finite measure, and <math>\nu</math> is a finite-valued signed or complex measure such that <math>\nu \ll \mu,</math> that is, <math>\nu</math> is [[absolutely continuous]] with respect to <math>\mu,</math> then there is a <math>\mu</math>-integrable real- or complex-valued function <math>g</math> on <math>X</math> such that for every measurable set <math>A,</math>
<math display=block>\nu(A) = \int_A g \, d\mu.</math>