Riemann’s Rearrangement Theorem

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 In 1827, Peter Lejeune-Dirichlet discovered this surprising result while working on conditions that ensured the convergence of the Fourier series. He was the first to notice that it is possible to rearrange the terms of certain series (now known as conditionally convergent series) so that the sum would change. Why is this result possible? 

*ma049.qxd (washington.edu)

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