Giuga number
In number theory, a Giuga number is a composite number n {\displaystyle n} such that for each of its distinct prime factors p i {\displaystyle p_{i}} we have p i | ( n p i − 1 ) {\displaystyle p_{i}|\left({n \over p_{i}}-1\right)} , or equivalently such that for each of its distinct prime factors pi we have p i 2 | ( n − p i ) {\displaystyle p_{i}^{2}|(n-p_{i})} . For example, 30 = 2 × 3 × 5 is a Giuga number since we can verify that: 30/2 − 1 = 14 = 2 × 7, 30/3 − 1 = 9 = 32, and 30/5 − 1 = 5.