# C* - Algebras and Numerical Analysis by Ronald Hagen, Steffen Roch, Bernd Silbermann

By Ronald Hagen, Steffen Roch, Bernd Silbermann

''Analyzes algebras of concrete approximation tools detailing must haves, neighborhood ideas, and lifting theorems. Covers fractality and Fredholmness. Explains the phenomena of the asymptotic splitting of the singular values, and more.''

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**Example text**

THE LANGUAGE OF NUMERICAL ANALYSIS Invertibility ators and Fredholmness of Toeplitz oper- A linear and bounded operator A on a Banach space X is invertible in L(X) if (and only if) its kernel is {0} and its range is all of X (Banach’s theorem). Thus, an operator A is invertible if (and only if) both its kernel Ker A and its cokernel CokerA := X/clos(ImA) are linear spaces with dimension zero. Coburn’s theorem simplifies this condition essentially in case T(a) is a non-zero Toeplitz operator. Indeed, in this case it is sufficient to know that the difference indT(a) := dimKerT(a) di mCokerT(a) is zer o in order to guarantee the invertibility of T(a).

Further, for n _> 0 set ~,~ := exp(2ri/(n 1)), an choose p ~ (0, 1). ,n. It is again obvious that Ln = L~ -+ I. 1. APPROXIMATION METHODS 31 which implies that Rn -~ I. ) Consequently, if K is compact on A2(]~) and A aI+K with so me a ¯ C\{0} then the collocation method (RnALn) is an approximation method for A, and this methodapplies if and only if A is invertible. 3 Finite section method The finite section methodis a special projection methodwhich we will only consider on a Hilbert space. /and normIlxll~/= (x, x).

Let, conversely, (A~) be an applicable approximation method for Then, by definition, the operators A~ are invertible for large n, and the sequence (A~IL~) of their inverses is strongly convergent. So, the BanachSteinhaus theorem (= the uniform boundedness principle) entails the stability of the sequence (An). In order to check the invertibility of A recall first that the definition of an applicable methodrequires the solvability of the equation Au = f for every right h~d side f. Hence, A is onto.