What is the Discrete Fourier Transform of a periodic sequence?

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Shor's algorithm utilises the properties of quantum computers to find $r$ in, $$ a^r = 1 \mod n $$ wherein $r$ is even and $a^{r/2} + 1\neq 0$. However I haven't been able to find any resources in how this relates to the Discrete Fourier Transform? As in how is the DFT of a sequence taken?