A Fermat pseudoprime to a base a, written psp(a), is a composite number n such that a^(n - 1) congruent 1 (mod n), i.e., it satisfies Fermat's little theorem. Sometimes the requirement that n must be odd is added which, for example would exclude 4 from being considered a psp(5). psp(2)s are called Poulet numbers or, less commonly, Sarrus numbers or Fermatians. The following table gives the first few Fermat pseudoprimes to some small bases b.

Carmichael number | Fermat's little theorem | Poulet number | pseudoprime