Problem 38 Hard Difficulty

Let $ f_n (x) = \left( \sin nx \right)/n^2. $ Show that the series $ \sum f_n(x) $ converges for all values of $ x $ but the series of derivatives $ \sum f'_n (x) $ diverges when $ x = 2n \pi, n $ an integer. For what values of $ x $ does the series $ \sum f''_n (x) $ converge?

Answer

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Video Transcript

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