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Fraňková–Helly selection theorem : ウィキペディア英語版
Fraňková–Helly selection theorem
In mathematics, the Fraňková–Helly selection theorem is a generalisation of Helly's selection theorem for functions of bounded variation to the case of regulated functions. It was proved in 1991 by the Czech mathematician Dana Fraňková.
==Background==

Let ''X'' be a separable Hilbert space, and let BV((''T'' ); ''X'') denote the normed vector space of all functions ''f'' : (''T'' ) → ''X'' with finite total variation over the interval (''T'' ), equipped with the total variation norm. It is well known that BV((''T'' ); ''X'') satisfies the compactness theorem known as Helly's selection theorem: given any sequence of functions (''f''''n'')''n''∈N in BV((''T'' ); ''X'') that is uniformly bounded in the total variation norm, there exists a subsequence
:\left( f_ \right) \subseteq (f_) \subset \mathrm((T ); X)
and a limit function ''f'' ∈ BV((''T'' ); ''X'') such that ''f''''n''(''k'')(''t'') converges weakly in ''X'' to ''f''(''t'') for every ''t'' ∈ (''T'' ). That is, for every continuous linear functional ''λ'' ∈ ''X''
*,
:\lambda \left( f_(t) \right) \to \lambda(f(t)) \mbox \mathbb \mbox k \to \infty.
Consider now the Banach space Reg((''T'' ); ''X'') of all regulated functions ''f'' : (''T'' ) → ''X'', equipped with the supremum norm. Helly's theorem does not hold for the space Reg((''T'' ); ''X''): a counterexample is given by the sequence
:f_ (t) = \sin (n t).
One may ask, however, if a weaker selection theorem is true, and the Fraňková–Helly selection theorem is such a result.

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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