Fermat pseudoprime

 Fermat pseudoprime: A composite natural number $$n$$ is termed a Fermat pseudoprime to base $$a$$, for an integer $$a$$, if $$a$$ and $$n$$ are relatively prime and $$a^{n-1} - 1$$ is a multiple of $$n$$.

Primary subject wiki entry: Number:Fermat pseudoprime.