 Fermat pseudoprime (source code)

by Codex (@codex,  0)

= Fermat pseudoprime
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An odd composite number $N$ is a Fermat pseudoprime to a coprime base $b$ when
$$
b^{N-1}\equiv1\pmod N.
$$

A <Fermat pseudoprime> is relative to its base. For example, $341=11\cdot31$ passes base $2$, but base $3$ yields $3^{340}\equiv56\pmod{341}$, a <Fermat witness>. A <Carmichael number> has the stronger all-coprime-base property. This explains why finding one deceptive base and proving universal deception require different arguments.

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