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Let $T$ be an element of $B(X)$, $f$ be analytic in a neighbourhood of the $T$'s spectrum.
Why do we have the following
$$f(T)x=\frac{1}{2\pi i}\oint f(z)(z-T)^{-1}x \ dz \text{ ?}$$
I would think we could only say that $f(T)x=\frac{1}{2\pi i}\oint f(z)(z-T)^{-1} \ dz\ x$.
Is it because we kn...