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Dirichlet distribution : ウィキペディア英語版
Dirichlet distribution

In probability and statistics, the Dirichlet distribution (after Peter Gustav Lejeune Dirichlet), often denoted \operatorname(\boldsymbol\alpha), is a family of continuous multivariate probability distributions parameterized by a vector \boldsymbol\alpha of positive reals. It is the multivariate generalization of the beta distribution.〔 (Chapter 49: Dirichlet and Inverted Dirichlet Distributions)〕 Dirichlet distributions are very often used as prior distributions in Bayesian statistics, and in fact the Dirichlet distribution is the conjugate prior of the categorical distribution and multinomial distribution. That is, its probability density function returns the belief that the probabilities of ''K'' rival events are x_i given that each event has been observed \alpha_i-1 times.
The infinite-dimensional generalization of the Dirichlet distribution is the ''Dirichlet process''.
==Probability density function==

The Dirichlet distribution of order ''K'' ≥ 2 with parameters ''α''1, ..., ''α''''K'' > 0 has a probability density function with respect to Lebesgue measure on the Euclidean space R''K''−1 given by
:f \left(x_1,\cdots, x_; \alpha_1,\cdots, \alpha_K \right) = \frac \prod_^K x_i^,
on the open (''K'' − 1)-dimensional simplex defined by:
:\begin
&x_1, \cdots, x_ > 0 \\
&x_1 + \cdots + x_ < 1 \\
&x_K = 1 - x_1 - \cdots - x_
\end
and zero elsewhere.
The normalizing constant is the multinomial Beta function, which can be expressed in terms of the gamma function:
:\mathrm(\boldsymbol\alpha) = \frac,\qquad\boldsymbol=(\alpha_1,\cdots,\alpha_K).

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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