Giuga number

A Giuga number is a composite number n such that for each of its distinct prime factors p_i we have ${\displaystyle p_{i}|\left({n \over p_{i}}-1\right)}$, or equivalently such that for each of its distinct prime factors p_i we have ${\displaystyle p_{i}^{2}|(n-p_{i})}$.

The Giuga numbers are named after the mathematician Giuseppe Giuga, and relate to his conjecture on primality.