A Giuga number is a composite number n such that for each of its distinct prime factors p_{i} we have , or equivalently such that for each of its distinct prime factors p_{i} we have .
The Giuga numbers are named after the mathematician Giuseppe Giuga, and relate to his conjecture on primality.
Definitions
Alternative definition for a Giuga number due to Takashi Agoh is: a composite number n is a Giuga number if and only if the congruence
holds true, where B is a Bernoulli number and is Euler's totient function.
An equivalent formulation due to Giuseppe Giuga is: a composite number n is a Giuga number if and only if the congruence
and if and only if
All known Giuga numbers n in fact satisfy the stronger condition
Examples
The sequence of Giuga numbers begins
For example, 30 is a Giuga number since its prime factors are 2, 3 and 5, and we can verify that
 30/2  1 = 14, which is divisible by 2,
 30/3  1 = 9, which is 3 squared, and
 30/5  1 = 5, the third prime factor itself.
Properties
The prime factors of a Giuga number must be distinct. If divides , then it follows that , where is divisible by . Hence, would not be divisible by , and thus would not be a Giuga number.
Thus, only squarefree integers can be Giuga numbers. For example, the factors of 60 are 2, 2, 3 and 5, and 60/2  1 = 29, which is not divisible by 2. Thus, 60 is not a Giuga number.
This rules out squares of primes, but semiprimes cannot be Giuga numbers either. For if , with primes, then , so will not divide , and thus is not a Giuga number.
Unsolved problem in mathematics: Are there infinitely many Giuga numbers? (more unsolved problems in mathematics)

All known Giuga numbers are even. If an odd Giuga number exists, it must be the product of at least 14 primes. It is not known if there are infinitely many Giuga numbers.
It has been conjectured by Paolo P. Lava (2009) that Giuga numbers are the solutions of the differential equation n' = n+1, where n' is the arithmetic derivative of n. (For squarefree numbers , , so n' = n+1 is just the last equation in the above section Definitions, multiplied by n.)
José Mª Grau and Antonio OllerMarcén have shown that an integer n is a Giuga number if and only if it satisfies n' = a n + 1 for some integer a > 0, where n' is the arithmetic derivative of n. (Again, n' = a n + 1 is identical to the third equation in Definitions, multiplied by n.)
See also
References
 Weisstein, Eric W. "Giuga Number". MathWorld.
 Borwein, D.; Borwein, J. M.; Borwein, P. B.; Girgensohn, R. (1996). "Giuga's Conjecture on Primality" (PDF). American Mathematical Monthly. 103 (1): 40–50. CiteSeerX 10.1.1.586.1424. doi:10.2307/2975213. JSTOR 2975213. Zbl 0860.11003. Archived from the original (PDF) on 20050531.
 Balzarotti, Giorgio; Lava, Paolo P. (2010). Centotre curiosità matematiche. Milan: Hoepli Editore. p. 129. ISBN 9788820345563.