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beta function : ウィキペディア英語版
beta function

In mathematics, the beta function, also called the Euler integral of the first kind, is a special function defined by
:
\mathrm(x,y) = \int_0^1t^(1-t)^\,\mathrmt
\!
for \textrm(x), \textrm(y) > 0.\,

The beta function was studied by Euler and Legendre and was given its name by Jacques Binet; its symbol Β is a Greek capital β rather than the similar Latin capital B.
== Properties ==
The beta function is symmetric, meaning that
:
\Beta(x,y) = \Beta(y,x).
\!〔Davis (1972) 6.2.2 p.258〕
When ''x'' and ''y'' are positive integers, it follows from the definition of the gamma function \Gamma\ that:
:
\Beta(x,y)=\dfrac
\!
It has many other forms, including:
:
\Beta(x,y)=\dfrac
\!〔
:
\Beta(x,y) =
2\int_0^(\sin\theta)^(\cos\theta)^\,\mathrm\theta,
\qquad \mathrm(x)>0,\ \mathrm(y)>0
\!〔
:
\Beta(x,y) =
\int_0^\infty\dfrac}\,\mathrmt,
\qquad \mathrm(x)>0,\ \mathrm(y)>0
\!〔Davis (1972) 6.2.1 p.258〕
:
\Beta(x,y) =
\sum_^\infty \dfrac ,
\!
:
\Beta(x,y) = \frac \prod_^\infty \left( 1+ \dfrac\right)^,
\!
The Beta function satisfies several interesting identities, including
:
\Beta(x,y) = \Beta(x, y+1) + \Beta(x+1, y)
\!
:
\Beta(x+1,y) = \Beta(x, y) \cdot \dfrac
\!
:
\Beta(x,y+1) = \Beta(x, y) \cdot \dfrac
\!
:
\Beta(x,y)\cdot(t \mapsto t_+^) = (t \to t_+^)
* (t \to t_+^) \qquad x\ge 1, y\ge 1,
\!
:
\Beta(x,y) \cdot \Beta(x+y,1-y) =
\dfrac,
\!

where t \mapsto t_+^x is a truncated power function and the star denotes convolution.

The lowermost identity above shows in particular \Gamma(\tfrac12) = \sqrt \pi. Some of these identities, e.g. the trigonometric formula, can be applied to deriving the volume of an n-ball in Cartesian coordinates.
Euler's integral for the beta function may be converted into an integral over the Pochhammer contour ''C'' as
:\displaystyle (1-e^)(1-e^)\Beta(\alpha,\beta) =\int_C t^(1-t)^ \, \mathrmt.
This Pochhammer contour integral converges for all values of ''α'' and ''β'' and so gives the analytic continuation of the beta function.
Just as the gamma function for integers describes factorials, the beta function can define a binomial coefficient after adjusting indices:
: = \frac1.
Moreover, for integer ''n'', \Beta\, can be factored to give a closed form, an interpolation function for continuous values of ''k'':
: = (-1)^n n! \cfrac.
The beta function was the first known scattering amplitude in string theory, first conjectured by Gabriele Veneziano. It also occurs in the theory of the preferential attachment process, a type of stochastic urn process.

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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