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Yule–Simon distribution : ウィキペディア英語版
Yule–Simon distribution
\, for \rho>3|
kurtosis =\rho+3+\frac for \rho>4|
entropy =|
mgf =\frac|
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In probability and statistics, the Yule–Simon distribution is a discrete probability distribution named after Udny Yule and Herbert A. Simon. Simon originally called it the ''Yule distribution''.
The probability mass function (pmf) of the Yule–Simon (''ρ'') distribution is
:f(k;\rho) = \rho\operatorname(k, \rho+1),
for integer k \geq 1 and real \rho > 0, where \operatorname is the beta function. Equivalently the pmf can be written in terms of the falling factorial as
:
f(k;\rho) = \frac
,
where \Gamma is the gamma function. Thus, if \rho is an integer,
:
f(k;\rho) = \frac
.
The parameter \rho can be estimated using a fixed point algorithm.
The probability mass function ''f'' has the property that for sufficiently large ''k'' we have
:
f(k;\rho)
\approx \frac{k^{\rho+1}}
.
This means that the tail of the Yule–Simon distribution is a realization of Zipf's law: f(k;\rho) can be used to model, for example, the relative frequency of the kth most frequent word in a large collection of text, which according to Zipf's law is inversely proportional to a (typically small) power of k.
==Occurrence==

The Yule–Simon distribution arose originally as the limiting distribution of a particular stochastic process studied by Yule as a model for the distribution of biological taxa and subtaxa.〔


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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