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

~~\frac}}\left(\sqrt}\right)|
mean =\infty|
median =c/2(\textrm^(1/2))^2\,, for \mu=0|
mode =\frac, for \mu=0|
variance =\infty|
skewness =undefined|
kurtosis =undefined|
entropy =\frac
where \gamma is Euler's constant|
mgf =undefined|
char =e^}
In probability theory and statistics, the Lévy distribution, named after Paul Lévy, is a continuous probability distribution for a non-negative random variable. In spectroscopy, this distribution, with frequency as the dependent variable, is known as a van der Waals profile.〔"van der Waals profile" appears with lowercase "van" in almost all sources, such as: ''Statistical mechanics of the liquid surface'' by Clive Anthony Croxton, 1980, A Wiley-Interscience publication, ISBN 0-471-27663-4, ISBN 978-0-471-27663-0, (); and in ''Journal of technical physics'', Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995, ()〕 It is a special case of the inverse-gamma distribution.
It is one of the few distributions that are stable and that have probability density functions that can be expressed analytically, the others being the normal distribution and the Cauchy distribution.
==Definition==

The probability density function of the Lévy distribution over the domain x\ge \mu is
:f(x;\mu,c)=\sqrt}~~\frac}} \left(\sqrt}\right)
where \textrm(z) is the complementary error function. The shift parameter \mu has the effect of shifting the curve to the right by an amount \mu, and changing the support to the interval [\mu, \infty). Like all stable distributions, the Levy distribution has a standard form f(x;0,1) which has the following property:
:f(x;\mu,c)dx = f(y;0,1)dy\,
where ''y'' is defined as
:y = \frac\,
The characteristic function of the Lévy distribution is given by
:\varphi(t;\mu,c)=e^~(1-i~\textrm(t))}.
Assuming \mu=0, the ''n''th moment of the unshifted Lévy distribution is formally defined by:
:m_n\ \stackrel\ \sqrt}\int_0^\infty \frac}\ \sqrt}\int_0^\infty \frac}\,dx
which diverges for t>0 and is therefore not defined in an interval around zero, so that the moment generating function is not defined ''per se''. Like all stable distributions except the normal distribution, the wing of the probability density function exhibits heavy tail behavior falling off according to a power law:
:\lim_f(x;\mu,c) =\sqrt}~\frac} \left(-\frac\right)^}\right\}


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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