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Hölder's theorem : ウィキペディア英語版
Hölder's theorem
In mathematics, Hölder's theorem states that the gamma function does not satisfy any algebraic differential equation whose coefficients are rational functions. The result was first proved by Otto Hölder in 1887; several alternative proofs have subsequently been found.〔Bank, Steven B. & Kaufman, Robert "(A Note on Hölder's Theorem Concerning the Gamma Function )". ''Mathematische Annalen'', vol 232, 1978.〕
The theorem also generalizes to the q-gamma function.
==Statement of the Theorem==
There is no non-constant polynomial P(x;\;y_0,\;y_1,\ldots,\;y_n) such that
: \,P\left(x;\;\Gamma(x),\;\Gamma'(x),\;\ldots\;,\;\Gamma^(x)\right)\equiv 0.\!
where y_0,\;y_1,\ldots,\;y_n, are functions of ''x'', Γ(''x'') is the gamma function, and ''P'' is a polynomial in y_0,\;y_1,\ldots,\;y_n, with coefficients drawn from the ring of polynomials in ''x''. That is,
: \, P(x;\;y_0,\;y_1,\ldots,\;y_n)=\sum_ A_(x)\cdot(y_0)^\cdot(y_1)^\cdot\ldots\cdot(y_n)^\!
where \,(a_0,\;a_1,\ldots,\;a_n)\, indexes all possible terms of the polynomial and \, A_(x)\, are polynomials in ''x'' acting as coefficients of the polynomial ''P''. The \, A_(x)\, may be constants or zero.
For example, if \, P(x;\; y_0, \;y_1,\; y_2)= x^2 y_2+x y_1+(x^2-\nu^2)y_0\, then \, A_(x)=x^2\,, \, A_(x)=x\, and \, A_(x)=(x^2-\nu^2)\, where ν is a constant. All the other coefficients in the summation are zero. Then
: \, P(z;\;f,\;f',\;f'')= x^2 f''+x f'+(x^2-\nu^2)f=0\,
is an ''algebraic differential equation'' which, in this example, has solutions \, f=J_\nu(x)\, and \, f=Y_\nu(x)\, , the Bessel functions of either the first or second kind. So
: \, P\left(x;\;J_\nu(x),\;J_\nu '(x),\;J_\nu ''(x)\right)\equiv 0.\!
and therefore both \, J_\nu(x) \, and \, Y_\nu(x)\, are ''differentially algebraic'' (also ''algebraically transcendental''). Most of the familiar special functions of mathematical physics are differentially algebraic. All algebraic combinations of differentially algebraic functions are also differentially algebraic. Also, all compositions of differentially algebraic functions are differentially algebraic. Hölder's Theorem simply states that the gamma function, Γ(''x'') is not differentially algebraic and is, therefore, ''transcendentally transcendental.''〔

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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