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Goldstine theorem : ウィキペディア英語版
Goldstine theorem
In functional analysis, a branch of mathematics, the Goldstine theorem, named after Herman Goldstine, is stated as follows:
:Goldstine Theorem. Let be a Banach space, then the image of the closed unit ball under the canonical embedding into the closed unit ball of the bidual space is weak
*
-dense.
The conclusion of the theorem is not true for the norm topology, which can be seen by considering the Banach space of real sequences that converge to zero, , and its bi-dual space .
== Proof ==
Given , an -tuple of linearly independent elements of and a we shall find in such that for .
If the requirement is dropped, the existence of such an follows from the surjectivity of
:\begin \Phi : X \to \mathbf^, \\ x \mapsto \left (\varphi_1(x), \cdots, \varphi_n(x) \right ) \end
Now let
:Y := \bigcap_i \ker \varphi_i = \ker \Phi.
Every element of has the required property, so that it suffices to show that the latter set is not empty.
Assume that it is empty. Then and by the Hahn-Banach theorem there exists a linear form such that and . Then and therefore
:1+\delta \leq \varphi(x) = x''(\varphi) \leq \|\varphi\|_ \left \|x'' \right \|_ \leq 1,
which is a contradiction.

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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