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Let $(X,\mathcal{F})$ be a measurable space and let $E:\mathcal{F}\to\mathscr{B(H)}$ be a spectral measure. Let $\phi\in B(X)$ be a simple function whose image is $\{\lambda_1,\ldots,\lambda_n\}\subset\mathbb{C}$, define

$\intop_X \phi dE = \sum_{i=1}^n\lambda_i\cdot E(\phi^{-1}(\lambda_i))$

Now, for a general $\phi\in B(X)$ let $\phi_n\to\phi$ uniformly, and define

$\intop_X \phi dE = \lim\intop_X \phi_n dE$

Prove that this integration operator is uniquely defined.

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    A limit is missing. Do you know what you have to show?2012-06-09
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    Yeah, I've actually managed to since I've posted this question...2012-06-09
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    Well, first show that the limit indeed exist, then that it doesn't depend on the approximating sequence2012-06-09
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    In fact when we are stuck at a math exercise, it can be for two reason: either we don't know what we have to show, or we know it but we don't see what it will be true. I asked you in which of these two cases you were in order to give the best help I can.2012-06-09
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    -1: Surely you know by now that asking a homework question calls for more than just copying the problem.2012-06-09

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