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Fermat's little theorem: $n^p\equiv n \; (mod \;p)$ for $p$ prime and all integers $n$.
Synopsis of proof: Reduce to nontrivial case where $p$ doesn't divide $n$, interpret as equality in field of $p$ elements, divide by $n$ and apply Lagrange's theorem saying that the order of a finite group kills all its elements.
Fermat's theorem: $n^p\equiv n \; (mod \;p)$ for $p$ prime and all integers $n$.
Synopsis of proof: Reduce to nontrivial case where $p$ doesn't divide $n$, interpret as equality in field of $p$ elements, divide by $n$ and apply Lagrange's theorem saying that the order of a finite group kills all its elements.