# Unitary divisor

In mathematics, a natural number a is a unitary divisor of a number b if a is a divisor of b and if a and $\frac{b}{a}$ are coprime, having no common factor other than 1. Thus, 5 is a unitary divisor of 60, because 5 and $\frac{60}{5}=12$ have only 1 as a common factor, while 6 is a divisor but not a unitary divisor of 60, as 6 and $\frac{60}{6}=10$ have a common factor other than 1, namely 2. 1 is a unitary divisor of every natural number.

Equivalently, a given divisor a of b is a unitary divisor iff every prime factor of a has the same multiplicity in a as it has in b.

The sum of unitary divisors function is denoted by the lowercase Greek letter sigma thus: σ*(n). The sum of the k-th powers of the unitary divisors is denoted by σ*k(n):

$\sigma_k^*(n) = \sum_{d\mid n \atop \gcd(d,n/d)=1} \!\! d^k.$

If the proper unitary divisors of a given number add up to that number, then that number is called a unitary perfect number.

## Properties

The number of unitary divisors of a number n is 2k, where k is the number of distinct prime factors of n. The sum of the unitary divisors of n is odd if n is a power of 2 (including 1), and even otherwise.

Both the count and the sum of the unitary divisors of n are multiplicative functions of n that are not completely multiplicative. The Dirichlet generating function is

$\frac{\zeta(s)\zeta(s-k)}{\zeta(2s-k)} = \sum_{n\ge 1}\frac{\sigma_k^*(n)}{n^s}.$

## Odd unitary divisors

The sum of the k-th powers of the odd unitary divisors is

$\sigma_k^{(o)*}(n) = \sum_{{d\mid n \atop d\equiv 1 \pmod 2} \atop \gcd(d,n/d)=1} \!\! d^k.$

It is also multiplicative, with Dirichlet generating function

$\frac{\zeta(s)\zeta(s-k)(1-2^{k-s})}{\zeta(2s-k)(1-2^{k-2s})} = \sum_{n\ge 1}\frac{\sigma_k^{(o)*}(n)}{n^s}.$

## Bi-unitary divisors

A divisor d of n is a bi-unitary divisor if the greatest common unitary divisor of d and n/d is 1. The number of bi-unitary divisors of n is a multiplicative function of n with average order $A \log x$ where[1]

$A = \prod_p\left({1 - \frac{p-1}{p^2(p+1)} }\right) \ .$

A bi-unitary perfect number is one equal to the sum of its bi-unitary aliquot divisors. The only such numbers are 6, 60 and 90.[2]

## References

1. ^ Ivić (1985) p.395
2. ^ Sandor et al (2006) p.115