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Riesz sequence : ウィキペディア英語版
Riesz sequence
In mathematics, a sequence of vectors (''x''''n'') in a Hilbert space (H,\langle\cdot,\cdot\rangle) is called a Riesz sequence if there exist constants 0 such that
: c\left( \sum_n | a_n|^2 \right) \leq \left\Vert \sum_n a_n x_n \right\Vert^2 \leq C \left( \sum_n | a_n|^2 \right)
for all sequences of scalars (''a''''n'') in the ''p'' space2. A Riesz sequence is called a Riesz basis if
:\overline = H .
==Theorems==
If ''H'' is a finite-dimensional space, then every basis of ''H'' is a Riesz basis.
Let \varphi be in the ''L''''p'' space ''L''2(R), let
:\varphi_n(x) = \varphi(x-n)
and let \hat denote the Fourier transform of ''φ''. Define constants ''c'' and ''C'' with 0. Then the following are equivalent:
:1. \quad \forall (a_n) \in \ell^2,\ \ c\left( \sum_n | a_n|^2 \right) \leq \left\Vert \sum_n a_n \varphi_n \right\Vert^2 \leq C \left( \sum_n | a_n|^2 \right)
:2. \quad c\leq\sum_\left|\hat(\omega + 2\pi n)\right|^2\leq C
The first of the above conditions is the definition for (''φ''''n'') to form a Riesz basis for the space it spans.

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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