[math] \newcommand{\mathds}{\mathbb}[/math]

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Prove the following formula, with [math]w=e^{2\pi i/N}[/math],

[[math]] \frac{1}{N}\sum_kw^{jk}=\delta_{0j} [[/math]]

where all the indices, and the Kronecker symbol too, are taken modulo [math]N[/math].