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Polynomially reflexive space : ウィキペディア英語版
Polynomially reflexive space
In mathematics, a polynomially reflexive space is a Banach space ''X'', on which the space of all polynomials in each degree is a reflexive space.
Given a multilinear functional ''M''''n'' of degree ''n'' (that is, ''M''''n'' is ''n''-linear), we can define a polynomial ''p'' as
:p(x)=M_n(x,\dots,x)
(that is, applying ''M''''n'' on the ''diagonal'') or any finite sum of these. If only ''n''-linear functionals are in the sum, the polynomial is said to be ''n''-homogeneous.
We define the space ''P''''n'' as consisting of all ''n''-homogeneous polynomials.
The ''P''1 is identical to the dual space, and is thus reflexive for all reflexive ''X''. This implies that reflexivity is a prerequisite for polynomial reflexivity.
==Relation to continuity of forms==

On a finite-dimensional linear space, a quadratic form ''x''↦''f''(''x'') is always a (finite) linear combination of products ''x''↦''g''(''x'') ''h''(''x'') of two linear functionals ''g'' and ''h''. Therefore, assuming that the scalars are complex numbers, every sequence ''xn'' satisfying ''g''(''xn'') → 0 for all linear functionals ''g'', satisfies also ''f''(''xn'') → 0 for all quadratic forms ''f''.
In infinite dimension the situation is different. For example, in a Hilbert space, an orthonormal sequence ''xn'' satisfies ''g''(''xn'') → 0 for all linear functionals ''g'', and nevertheless ''f''(''xn'') = 1 where ''f'' is the quadratic form ''f''(''x'') = ||''x''||2. In more technical words, this quadratic form fails to be weakly sequentially continuous at the origin.
On a reflexive Banach space with the approximation property the following two conditions are equivalent:〔Farmer 1994, page 261.〕
* every quadratic form is weakly sequentially continuous at the origin;
* the Banach space of all quadratic forms is reflexive.
Quadratic forms are 2-homogeneous polynomials. The equivalence mentioned above holds also for ''n''-homogeneous polynomials, ''n''=3,4,...

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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