Editing Radon–Nikodym theorem (section)

===Radon–Nikodym derivative===
The function <math>f</math> satisfying the above equality is {{em|uniquely defined [[up to]] a <math>\mu</math>-[[null set]]}}, that is, if <math>g</math> is another function which satisfies the same property, then <math>f = g</math> {{nowrap|<math>\mu</math>-[[almost everywhere]]}}. The function <math>f</math> is commonly written <math display>\frac{d\nu}{d\mu}</math> and is called the '''{{visible anchor|Radon–Nikodym derivative}}'''. The choice of notation and the name of the function reflects the fact that the function is analogous to a [[derivative]] in [[calculus]] in the sense that it describes the rate of change of density of one measure with respect to another (the way the [[Jacobian determinant]] is used in multivariable integration).