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Quotient of subspace theorem : ウィキペディア英語版
Quotient of subspace theorem
In mathematics, the quotient of subspace theorem is an important property of finite-dimensional normed spaces, discovered by Vitali Milman.〔The original proof appeared in . See also .〕
Let (''X'', ||·||) be an ''N''-dimensional normed space. There exist subspaces ''Z'' ⊂ ''Y'' ⊂ ''X'' such that the following holds:
* The quotient space ''E'' = ''Y'' / ''Z'' is of dimension dim E ≥ ''c'' ''N'', where ''c'' > 0 is a universal constant.
* The induced norm || · || on ''E'', defined by
:: \| e \| =\min_ \| y \|, \quad e \in E,
is uniformly isomorphic to Euclidean. That is, there exists a positive quadratic form ("Euclidean structure") ''Q'' on ''E'', such that
:: \frac \leq \| e \| \leq K \sqrt for e \in E,
with ''K'' > 1 a universal constant.
The statement is relative easy to prove by induction on the dimension of ''Z'' (even for ''Y=Z'', ''X''=''0'', ''c=1'') with a ''K'' that depends only on ''N''; the point of the theorem is that ''K'' is independent of ''N''.
In fact, the constant ''c'' can be made arbitrarily close to 1, at the expense of the
constant ''K'' becoming large. The original proof allowed
: c(K) \approx 1 - \text / \log \log K. 〔See references for improved estimates.〕
==Notes==


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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