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

The (one-dimensional) Holtsmark distribution is a continuous probability distribution. The Holtsmark distribution is a special case of a stable distribution with the index of stability or shape parameter \alpha equal to 3/2 and skewness parameter \beta of zero. Since \beta equals zero, the distribution is symmetric, and thus an example of a symmetric alpha-stable distribution. The Holtsmark distribution is one of the few examples of a stable distribution for which a closed form expression of the probability density function is known. However, its probability density function is not expressible in terms of elementary functions; rather, the probability density function is expressed in terms of hypergeometric functions.
The Holtsmark distribution has applications in plasma physics and astrophysics.〔 In 1919, Norwegian physicist J. Holtsmark proposed the distribution as a model for the fluctuating fields in plasma due to chaotic motion of charged particles. It is also applicable to other types of Coulomb forces, in particular to modeling of gravitating bodies, and thus is important in astrophysics.
==Characteristic function==

The characteristic function of a symmetric stable distribution is:
:
\varphi(t;\mu,c) =
\exp\left(t|^\alpha~\right ),

where \alpha is the shape parameter, or index of stability, \mu is the location parameter, and ''c'' is the scale parameter.
Since the Holtsmark distribution has \alpha=3/2, its characteristic function is:
:
\varphi(t;\mu,c) =
\exp\left(t|^~\right ) .

Since the Holtsmark distribution is a stable distribution with , \mu represents the mean of the distribution. Since , \mu also represents the median and mode of the distribution. And since , the variance of the Holtsmark distribution is infinite.〔 All higher moments of the distribution are also infinite.〔 Like other stable distributions (other than the normal distribution), since the variance is infinite the dispersion in the distribution is reflected by the scale parameter, c. An alternate approach to describing the dispersion of the distribution is through fractional moments.〔

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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