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In mathematical analysis, the Hilbert–Schmidt theorem, also known as the eigenfunction expansion theorem, is a fundamental result concerning compact, self-adjoint operators on Hilbert spaces. In the theory of partial differential equations, it is very useful in solving elliptic boundary value problems. ==Statement of the theorem== Let (''H'', 〈 , 〉) be a real or complex Hilbert space and let ''A'' : ''H'' → ''H'' be a bounded, compact, self-adjoint operator. Then there is a sequence of non-zero real eigenvalues ''λ''''i'', ''i'' = 1, ..., ''N'', with ''N'' equal to the rank of ''A'', such that |''λ''''i''| is monotonically non-increasing and, if ''N'' = +∞, : Furthermore, if each eigenvalue of ''A'' is repeated in the sequence according to its multiplicity, then there exists an orthonormal set ''φ''''i'', ''i'' = 1, ..., ''N'', of corresponding eigenfunctions, i.e. : Moreover, the functions ''φ''''i'' form an orthonormal basis for the range of ''A'' and ''A'' can be written as : 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Hilbert–Schmidt theorem」の詳細全文を読む スポンサード リンク
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