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In operator algebras, the Toeplitz algebra is the C *-algebra generated by the unilateral shift on the Hilbert space ''l''2(N). Taking ''l''2(N) to be the Hardy space ''H''2, the Toeplitz algebra consists of elements of the form : where ''Tf'' is a Toeplitz operator with continuous symbol and ''K'' is a compact operator. Toeplitz operators with continuous symbols commute modulo the compact operators. So the Toeplitz algebra can be viewed as the C *-algebra extension of continuous functions on the circle by the compact operators. This extension is called the Toeplitz extension. By Atkinson's theorem, an element of the Toeplitz algebra ''Tf'' + ''K'' is a Fredholm operator if and only if the symbol ''f'' of ''Tf'' is invertible. In that case, the Fredholm index of ''Tf'' + ''K'' is precisely the winding number of ''f'', the equivalence class of ''f'' in the fundamental group of the circle. This is a special case of the Atiyah-Singer index theorem. Wold decomposition characterizes proper isometries acting on a Hilbert space. From this, together with properties of Toeplitz operators, one can conclude that the Toeplitz algebra is the universal C *-algebra generated by a proper isometry; this is ''Coburn's theorem''. == References == 〔 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「Toeplitz algebra」の詳細全文を読む スポンサード リンク
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