Giuga number

In number theory, a Giuga number is a composite number ${\displaystyle n}$ such that for each of its distinct prime factors ${\displaystyle p_{i}}$ 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 ${\displaystyle p_{i}^{2}|(n-p_{i})}$. 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.

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

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

${\displaystyle nB_{\varphi (n)}\equiv -1{\pmod {n}}}$

holds true, where B is a Bernoulli number and ${\displaystyle \varphi (n)}$ 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

${\displaystyle \sum _{i=1}^{n-1}i^{\varphi (n)}\equiv -1{\pmod {n}}}$

and if and only if

${\displaystyle \sum _{p|n}{\frac {1}{p}}-\prod _{p|n}{\frac {1}{p}}\in \mathbb {N} .}$

All known Giuga numbers n in fact satisfy the stronger condition

${\displaystyle \sum _{p|n}{\frac {1}{p}}-\prod _{p|n}{\frac {1}{p}}=1.}$

Thirteen Giuga numbers are known. The list is complete up to the 12th term and for numbers with 8 or fewer prime factors, but it is unknown if there is a Giuga number between the 12th and 13th terms.^[1]

The prime factors of a Giuga number must be distinct. If ${\displaystyle p^{2}}$ divides ${\displaystyle n}$, then it follows that ${\displaystyle {n \over p}-1=m-1}$, where ${\displaystyle m=n/p}$ is divisible by ${\displaystyle p}$. Hence, ${\displaystyle m-1}$ would not be divisible by ${\displaystyle p}$, and thus ${\displaystyle n}$ would not be a Giuga number.

Thus, only square-free 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 ${\displaystyle n=p_{1}p_{2}}$, with ${\displaystyle p_{1}<p_{2}}$ primes, then ${\displaystyle {n \over p_{2}}-1=p_{1}-1<p_{2}}$, so ${\displaystyle p_{2}}$ will not divide ${\displaystyle {n \over p_{2}}-1}$, and thus ${\displaystyle n}$ is not a Giuga number.

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 square-free numbers ${\displaystyle n=\prod _{i}{p_{i}}}$, ${\displaystyle n'=\sum _{i}{\frac {n}{p_{i}}}}$, so n' = n+1 is just the last equation in the above section Definitions, multiplied by n.)

José Mª Grau and Antonio Oller-Marcén 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.)

• Carmichael number
• Primary pseudoperfect number
• Znám's problem

• 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 2005-05-31.
• Balzarotti, Giorgio; Lava, Paolo P. (2010). Centotre curiosità matematiche. Milan: Hoepli Editore. p. 129. ISBN 978-88-203-4556-3.