Any composite number n with p|(n/p - 1) for all prime divisors p of n. n is a Giuga number iff sum_(k = 1)^(n - 1) k^(ϕ(n)) congruent -1 (mod n) where ϕ is the totient function and iff sum_(p|n) 1/p - product_(p|n) 1/p element N.

Agoh's conjecture | Bernoulli number | primary pseudoperfect number | totient function