Radon-Nikodym theorem

In mathematics, the Radon-Nikodym theorem is a result in functional analysis that states that if a measure is absolutely continuous with respect to another measure then there is a locally integrable function f on the underlying space such that

The function f is commonly written and is called the 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 probability density of one measure with respect to another. It follows trivially from the definition of the derivative that

where is the expectation operator.

The theorem is named for Johann Radon, who proved the theorem for the special case where the underlying space is in 1913, and for Otto Nikodym who proved the general case in 1930.