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Advanced Calculus fi..

Advanced Calculus fi..

Advanced Calculus fi..

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Chapter 7 Fourier Series and Orthogonal Functions 489Proof. Let the"3um of the Fourier series of f (x) be denoted by <strong>fi</strong> (x):Since the series converges uniformly, it follows from Theorem 1 that <strong>fi</strong>(x) is continuousand that a,, bn are the Fourier coef<strong>fi</strong>cients of <strong>fi</strong> (x). But the series was given asthe Fourier series of f (x). Hence f (x) and f, (x) have the same Fourier coef<strong>fi</strong>cients,and by Theorem 3, f (x) = fl(x); that is, f (x) is the sum of its Fourier series for-n Fx_

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