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{{for|the more general family of Lévy alpha-stable distributions, of which this distribution is a special case|stable distribution}}
{{Probability distribution|
  name      =Lévy (unshifted)|
  type      =density|
  pdf_image  =[[Image:Levy0 distributionPDF.svg|325px|Levy distribution PDF]]<br /><small></small>|
  cdf_image  =[[Image:Levy0 distributionCDF.svg|325px|Levy distribution CDF]]<br /><small></small>|
  parameters =<math>\mu</math> location; <math>c > 0\,</math> [[scale parameter|scale]]|
  support    =<math>x \in [\mu, \infty)</math>|
  pdf        =<math>\sqrt{\frac{c}{2\pi}}~~\frac{e^{-\frac{c}{2(x-\mu)}}}{(x-\mu)^{3/2}}</math>|
  cdf        =<math>\textrm{erfc}\left(\sqrt{\frac{c}{2(x-\mu)}}\right)</math>|
  mean      =<math>\infty</math>|
  median    =<math>c/2(\textrm{erfc}^{-1}(1/2))^2\,</math>, for <math>\mu=0</math>|
  mode      =<math>\frac{c}{3}</math>, for <math>\mu=0</math>|
  variance  =<math>\infty</math>|
  skewness  =undefined|
  kurtosis  =undefined|
  entropy    =<math>\frac{1+3\gamma+\ln(16\pi c^2)}{2}</math>
where <math>\gamma</math> is [[Euler's constant]]|
  mgf        =undefined|
  char      =<math>e^{i\mu t-\sqrt{-2ict}}</math>|
}}
In [[probability theory]] and [[statistics]], the '''Lévy distribution''', named after [[Paul Lévy (mathematician)|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]]'''.<ref group="note">"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, [http://books.google.it/books?id=Wve2AAAAIAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en]; and in ''Journal of technical physics'', Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995, [http://books.google.it/books?id=2XpVAAAAMAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en]<!-- and many more --></ref> It is a special case of the [[inverse-gamma distribution]].
 
It is one of the few distributions that are [[stability (probability)|stable]] and that have [[probability density function]]s that can be expressed analytically, the others being the [[normal distribution]] and the [[Cauchy distribution]]. All three are special cases of the [[stable distributions]], which do not generally have a probability density function which can be expressed analytically.
 
==Definition==
 
The [[probability density function]] of the Lévy distribution over the domain <math>x\ge \mu</math> is
 
:<math>f(x;\mu,c)=\sqrt{\frac{c}{2\pi}}~~\frac{e^{ -\frac{c}{2(x-\mu)}}} {(x-\mu)^{3/2}}</math>
 
where <math>\mu</math> is the [[location parameter]] and <math>c</math> is the [[scale parameter]]. The cumulative distribution function is
 
:<math>F(x;\mu,c)=\textrm{erfc}\left(\sqrt{\frac{c}{2(x-\mu)}}\right)</math>
 
where <math>\textrm{erfc}(z)</math> is the complementary [[error function]]. The shift parameter <math>\mu</math> has the effect of shifting the curve to the right by an amount <math>\mu</math>, and changing the support to the interval [<math>\mu</math>, <math>\infty</math>). Like all [[stable distribution]]s, the Levy distribution has a standard form f(x;0,1) which has the following property:
 
:<math>f(x;\mu,c)dx = f(y;0,1)dy\,</math>
 
where ''y'' is defined as
 
:<math>y = \frac{x-\mu}{c}\,</math>
 
The [[characteristic function (probability theory)|characteristic function]] of the Lévy distribution is given by
 
:<math>\varphi(t;\mu,c)=e^{i\mu t-\sqrt{-2ict}}.</math>
 
Note that the characteristic function can also be written in the same form used for the stable distribution with <math>\alpha=1/2</math> and <math>\beta=1</math>:
 
:<math>\varphi(t;\mu,c)=e^{i\mu t-|ct|^{1/2}~(1-i~\textrm{sign}(t))}.</math>
 
Assuming <math>\mu=0</math>, the ''n''th [[moment (mathematics)|moment]] of the unshifted Lévy distribution is formally defined by:
 
:<math>m_n\ \stackrel{\mathrm{def}}{=}\ \sqrt{\frac{c}{2\pi}}\int_0^\infty \frac{e^{-c/2x}\,x^n}{x^{3/2}}\,dx</math>
 
which diverges for all ''n''&nbsp;>&nbsp;0 so that the moments of the Lévy distribution do not exist. The [[moment generating function]] is then formally defined by:
 
:<math>M(t;c)\ \stackrel{\mathrm{def}}{=}\  \sqrt{\frac{c}{2\pi}}\int_0^\infty \frac{e^{-c/2x+tx}}{x^{3/2}}\,dx</math>
 
which diverges for <math>t>0</math> and is therefore not defined in an interval around zero, so that the moment generating function is not defined ''per se''. Like all [[stable distribution]]s except the [[normal distribution]], the wing of the probability density function exhibits heavy tail behavior falling off according to a power law:
 
:<math>\lim_{x\rightarrow \infty}f(x;\mu,c) =\sqrt{\frac{c}{2\pi}}~\frac{1}{x^{3/2}}.</math>
 
This is illustrated in the diagram below, in which the probability density functions for various values of ''c'' and <math>\mu=0</math> are plotted on a log-log scale.
 
[[Image:Levy0 LdistributionPDF.svg|325px|thumb|left|Probability density function for the Lévy distribution on a log-log scale.]]
 
<br style="clear:both;" />
 
==Related distributions==
* If <math>X \sim \textrm{Levy}(\mu,c)\, </math> then <math> k X + b \sim \textrm{Levy}(k \mu + b ,k c) \,</math>
* If <math>X\,\sim\,\textrm{Levy}(0,c)</math> then <math>X\,\sim\,\textrm{Inv-Gamma}(\tfrac{1}{2},\tfrac{c}{2})</math> ([[inverse gamma distribution]])
* Lévy distribution is a special case of type 5 [[Pearson distribution]]
* If <math>Y\,\sim\,\textrm{Normal}(\mu,\sigma^2)</math> ([[Normal distribution]]) then <math>{(Y-\mu)}^{-2} \sim\,\textrm{Levy}(0,1/\sigma^2)</math>
* If <math>X \sim \textrm{Normal}(\mu,\tfrac{1}{\sqrt{\sigma}})\, </math> then <math>{(X-\mu)}^{-2} \sim \textrm{Levy}(0,\sigma)\,</math>
* If <math>X\,\sim\,\textrm{Levy}(\mu,c)</math> then <math>X\,\sim\,\textrm{Stable}(1/2,1,c,\mu) \,</math> ([[Stable distribution]])
* If <math>X\,\sim\,\textrm{Levy}(0,c)</math> then <math>X\,\sim\,\textrm{Scale-inv-}\chi^2(1,c)</math> ([[Scaled-inverse-chi-squared distribution]])
* If <math>X\,\sim\,\textrm{Levy}(\mu,c)</math> then <math>{(X-\mu)}^{-\tfrac{1}{2}} \sim\,\textrm{FoldedNormal}(0,1/\sqrt{c})</math> ([[Folded normal distribution]])
 
==Applications==
 
* The frequency of [[geomagnetic reversal]]s appears to follow a Lévy distribution<!-- for ref see that article -->
*The [[hitting time|time of hitting]] a single point <math>\alpha</math> (different from the starting point 0) by the [[Wiener process|Brownian motion]] has the Lévy distribution with <math>c=\alpha^2</math>. (For a Brownian motion with drift, this time may follow an [[inverse Gaussian distribution]], which has the Lévy distribution as a limit.)
 
* The length of the path followed by a photon in a turbid medium follows the Lévy distribution.<ref>Rogers, Geoffrey L, Multiple path analysis of reflectance from turbid media. ''Journal of the Optical Society of America A,'' '''25''':11, p 2879-2883 (2008).</ref>
 
* A [[Cauchy process]] can be defined as a [[Brownian motion]] [[subordinator (mathematics)|subordinated]] to a process associated with a Lévy distribution.<ref name=applebaum>{{cite web|title=Lectures on Lévy processes and Stochastic calculus, Braunschweig; Lecture 2: Lévy processes|url=http://www.applebaum.staff.shef.ac.uk/Brauns2notes.pdf|author=Applebaum, D.|pages=37–53|publisher=University of Sheffield}}</ref>
 
== Footnotes ==
{{Reflist|group="note"}}
 
== Notes ==
{{Reflist}}
 
== References ==
* {{cite web | title=Information on stable distributions| work= | url=http://academic2.american.edu/~jpnolan/stable/stable.html | accessdate=July 13, 2005}} - John P. Nolan's introduction to stable distributions, some papers on stable laws, and a free program to compute stable densities, cumulative distribution functions, quantiles, estimate parameters, etc. See especially [http://academic2.american.edu/~jpnolan/stable/chap1.pdf An introduction to stable distributions, Chapter 1]
 
== External links ==
* {{MathWorld|title=Lévy Distribution|urlname=LevyDistribution}}
* [http://www.gummy-stuff.org/Levy.htm Lévy and stock prices]
 
{{ProbDistributions|continuous-semi-infinite}}
 
{{DEFAULTSORT:Levy distribution}}
[[Category:Continuous distributions]]
[[Category:Probability distributions with non-finite variance]]
[[Category:Power laws]]
[[Category:Stable distributions]]
[[Category:Probability distributions]]

Latest revision as of 00:30, 3 October 2013

28 year-old Painting Investments Worker Truman from Regina, usually spends time with pastimes for instance interior design, property developers in new launch ec Singapore and writing. Last month just traveled to City of the Renaissance. Template:Probability distribution 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.[note 1] 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. All three are special cases of the stable distributions, which do not generally have a probability density function which can be expressed analytically.

Definition

The probability density function of the Lévy distribution over the domain xμ is

f(x;μ,c)=c2πec2(xμ)(xμ)3/2

where μ is the location parameter and c is the scale parameter. The cumulative distribution function is

F(x;μ,c)=erfc(c2(xμ))

where erfc(z) is the complementary error function. The shift parameter μ has the effect of shifting the curve to the right by an amount μ, and changing the support to the interval [μ, ). Like all stable distributions, the Levy distribution has a standard form f(x;0,1) which has the following property:

f(x;μ,c)dx=f(y;0,1)dy

where y is defined as

y=xμc

The characteristic function of the Lévy distribution is given by

φ(t;μ,c)=eiμt2ict.

Note that the characteristic function can also be written in the same form used for the stable distribution with α=1/2 and β=1:

φ(t;μ,c)=eiμt|ct|1/2(1isign(t)).

Assuming μ=0, the nth moment of the unshifted Lévy distribution is formally defined by:

mn=defc2π0ec/2xxnx3/2dx

which diverges for all n > 0 so that the moments of the Lévy distribution do not exist. The moment generating function is then formally defined by:

M(t;c)=defc2π0ec/2x+txx3/2dx

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:

limxf(x;μ,c)=c2π1x3/2.

This is illustrated in the diagram below, in which the probability density functions for various values of c and μ=0 are plotted on a log-log scale.

Probability density function for the Lévy distribution on a log-log scale.


Related distributions

Applications

  • The length of the path followed by a photon in a turbid medium follows the Lévy distribution.[1]

Footnotes

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Notes

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

References

External links



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Cite error: <ref> tags exist for a group named "note", but no corresponding <references group="note"/> tag was found

  1. Rogers, Geoffrey L, Multiple path analysis of reflectance from turbid media. Journal of the Optical Society of America A, 25:11, p 2879-2883 (2008).
  2. Template:Cite web