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Bessel's inequality : ウィキペディア英語版
Bessel's inequality
In mathematics, especially functional analysis, Bessel's inequality is a statement about the coefficients of an element x in a Hilbert space with respect to an orthonormal sequence.
Let H be a Hilbert space, and suppose that e_1, e_2, ... is an orthonormal sequence in H. Then, for any x in H one has
:\sum_^\left\vert\left\langle x,e_k\right\rangle \right\vert^2 \le \left\Vert x\right\Vert^2
where 〈•,•〉 denotes the inner product in the Hilbert space H. If we define the infinite sum
:x' = \sum_^\left\langle x,e_k\right\rangle e_k,
consisting of 'infinite sum' of vector resolute x in direction e_k, Bessel's inequality tells us that this series converges. One can think of it that there exists x' \in H which can be described in terms of potential basis e_1, e_2, ....
For a complete orthonormal sequence (that is, for an orthonormal sequence which is a basis), we have Parseval's identity, which replaces the inequality with an equality (and consequently x' with x).
Bessel's inequality follows from the identity:
:0 \le \left\| x - \sum_^n \langle x, e_k \rangle e_k\right\|^2 = \|x\|^2 - 2 \sum_^n |\langle x, e_k \rangle |^2 + \sum_^n | \langle x, e_k \rangle |^2 = \|x\|^2 - \sum_^n | \langle x, e_k \rangle |^2,
which holds for any natural ''n''.
==See also==

* Cauchy–Schwarz inequality

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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