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Wirtinger's representation and projection theorem : ウィキペディア英語版
Wirtinger's representation and projection theorem

In mathematics, Wirtinger's representation and projection theorem is a theorem proved by Wilhelm Wirtinger in 1932 in connection with some problems of approximation theory. This theorem gives the representation formula for the holomorphic subspace \left.\right. H_2 of the simple, unweighted holomorphic Hilbert space \left.\right. L^2 of functions square-integrable over the surface of the unit disc \left.\right.\ of the complex plane, along with a form of the orthogonal projection from \left.\right. L^2 to \left.\right. H_2 .
Wirtinger's paper contains the following theorem presented also in Joseph L. Walsh's well-known monograph

(p. 150) with a different proof. ''If'' \left.\right.\left. F(z)\right. ''is of the class'' \left.\right. L^2 on \left.\right. |z|<1 , ''i.e.
: \iint_|F(z)|^2 \, dS<+\infty,
''where \left.\right. dS is the area element, then the unique function \left.\right. f(z) of the holomorphic subclass H_2\subset L^2 , such that''
: \iint_|F(z)-f(z)|^2 \, dS
''is least, is given by
: f(z)=\frac1\pi\iint_F(\zeta)\frac,\quad |z|<1.
The last formula gives a form for the orthogonal projection from \left.\right. L^2 to \left.\right. H_2 . Besides, replacement of \left.\right. F(\zeta) by \left.\right. f(\zeta) makes it Wirtinger's representation for all f(z)\in H_2 . This is an analog of the well-known Cauchy integral formula with the square of the Cauchy kernel. Later, after the 1950s, a degree of the Cauchy kernel was called reproducing kernel, and the notation \left.\right. A^2_0 became common for the class \left.\right. H_2.
In 1948 Mkhitar Djrbashian extended Wirtinger's representation and projection to the wider, weighted Hilbert spaces \left.\right. A^2_\alpha of functions \left.\right. f(z) holomorphic in \left.\right.|z|<1, which satisfy the condition
: \|f\|_=\left\ \, dS\right\}^<+\infty\text\alpha\in(0,+\infty),
and also to some Hilbert spaces of entire functions. The extensions of these results to some weighted \left.\right. A^2_\omega spaces of functions holomorphic in \left.\right. |z|<1 and similar spaces of entire functions, the unions of which respectively coincide with ''all'' functions holomorphic in \left.\right. |z|<1 and the ''whole'' set of entire functions can be seen in.
==See also==

*

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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