In number theory, two integers  and  are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1. Consequently, any prime number that divides  does not divide , and vice versa. This is equivalent to their greatest common divisor (GCD) being 1. One says also  is prime to  or  is coprime with .

The numbers 8 and 9 are coprime, despite the fact that neither considered individually is a prime number, since 1 is their only  common divisor. On the other hand, 6 and 9 are not coprime, because they are both divisible by 3. The numerator and denominator of a reduced fraction are coprime, by definition.

Notation and testing 
When the integers  and  are coprime, the standard way of expressing this fact in mathematical notation is to indicate that their greatest common divisor is one, by the formula  or .  In their 1989 textbook Concrete Mathematics, Ronald Graham, Donald Knuth, and Oren Patashnik proposed an alternative notation  to indicate that  and  are relatively prime and that the term "prime" be used instead of coprime (as in  is prime to ).

A fast way to determine whether two numbers are coprime is given by the Euclidean algorithm and its faster variants such as binary GCD algorithm or Lehmer's GCD algorithm.

The number of integers coprime with a positive integer , between 1 and , is given by Euler's totient function, also known as Euler's phi function, .

A set of integers can also be called coprime if its elements share no common positive factor except 1. A stronger condition on a set of integers is pairwise coprime, which means that  and  are coprime for every pair  of different integers in the set. The set  is coprime, but it is not pairwise coprime since 2 and 4 are not relatively prime.

Properties 
The numbers 1 and −1 are the only integers coprime with every integer, and they are the only integers that are coprime with 0.

A number of conditions are equivalent to  and  being coprime:
No prime number divides both  and .
There exist integers  such that  (see Bézout's identity).
The integer  has a multiplicative inverse modulo , meaning that there exists an integer  such that . In ring-theoretic language,  is a unit in the ring  of integers modulo .
Every pair of congruence relations for an unknown integer , of the form  and , has a solution (Chinese remainder theorem); in fact the solutions are described by a single congruence relation modulo .
The least common multiple of  and  is equal to their product , i.e. .

As a consequence of the third point, if  and  are coprime and , then . That is, we may "divide by " when working modulo . Furthermore, if  are both coprime with , then so is their product  (i.e., modulo  it is a product of invertible elements, and therefore invertible); this also follows from the first point by Euclid's lemma, which states that if a prime number  divides a product , then  divides at least one of the factors .

As a consequence of the first point, if  and  are coprime, then so are any powers  and .

If  and  are coprime and  divides the product , then  divides . This can be viewed as a generalization of Euclid's lemma.

The two integers  and  are coprime if and only if the point with coordinates  in a Cartesian coordinate system would be  "visible" via an unobstructed line of sight from the origin , in the sense that there is no point with integer coordinates anywhere on the line segment  between the origin and . (See figure 1.)

In a sense that can be made precise, the probability that two randomly chosen integers are coprime is , which is about 61% (see , below).

Two natural numbers  and  are coprime if and only if the numbers  and  are coprime. As a generalization of this, following easily from the Euclidean algorithm in base :

Coprimality in sets 

A set of integers  can also be called coprime or setwise coprime if the greatest common divisor of all the elements of the set is 1. For example, the integers 6, 10, 15 are coprime because 1 is the only positive integer that divides all of them.

If every pair in a set of integers is coprime, then the set is said to be pairwise coprime (or pairwise relatively prime, mutually coprime or mutually relatively prime). Pairwise coprimality is a stronger condition than setwise coprimality; every pairwise coprime finite set is also setwise coprime, but the reverse is not true. For example, the integers 4, 5, 6 are (setwise) coprime (because the only positive integer dividing all of them is 1), but they are not pairwise coprime (because ).

The concept of pairwise coprimality is important as a hypothesis in many results in number theory, such as the Chinese remainder theorem.

It is possible for an infinite set of integers to be pairwise coprime. Notable examples include the set of all prime numbers, the set of elements in Sylvester's sequence, and the set of all Fermat numbers.

Coprimality in ring ideals 

Two ideals  and  in a commutative ring  are called coprime (or comaximal) if  This generalizes Bézout's identity: with this definition, two principal ideals () and () in the ring of integers  are coprime if and only if  and  are coprime. If the ideals  and  of  are coprime, then  furthermore, if  is a third ideal such that  contains , then  contains . The Chinese remainder theorem can be generalized to any commutative ring, using coprime ideals.

Probability of coprimality 

Given two randomly chosen integers  and , it is reasonable to ask how likely it is that  and  are coprime. In this determination, it is convenient to use the characterization that  and  are coprime if and only if no prime number divides both of them (see Fundamental theorem of arithmetic).

Informally, the probability that any number is divisible by a prime (or in fact any integer)  is  for example, every 7th integer is divisible by 7. Hence the probability that two numbers are both divisible by  is  and the probability that at least one of them is not is   Any finite collection of divisibility events associated to distinct primes is mutually independent.  For example, in the case of two events, a number is divisible by primes  and  if and only if it is divisible by ; the latter event has probability   If one makes the heuristic assumption that such reasoning can be extended to infinitely many divisibility events, one is led to guess that the probability that two numbers are coprime is given by a product over all primes,

 

Here  refers to the Riemann zeta function, the identity relating the product over primes to  is an example of an Euler product, and the evaluation of  as  is the Basel problem, solved by Leonhard Euler in 1735.

There is no way to choose a positive integer at random so that each positive integer occurs with equal probability, but statements about "randomly chosen integers" such as the ones above can be formalized by using the notion of natural density.  For each positive integer , let  be the probability that two randomly chosen numbers in  are coprime.   Although  will never equal  exactly, with work one can show that in the limit as  the probability  approaches .

More generally, the probability of  randomly chosen integers being coprime is

Generating all coprime pairs

All pairs of positive coprime numbers  (with ) can be arranged in two disjoint complete ternary trees, one tree starting from  (for even–odd and odd–even pairs), and the other tree starting from  (for odd–odd pairs). The children of each vertex  are generated as follows:
Branch 1: 
Branch 2: 
Branch 3: 

This scheme is exhaustive and non-redundant with no invalid members. This can be proved by remarking that, if  is a coprime pair with  then 
if  then   is a child of  along branch 3;
if  then  is a child of  along branch 2;
if  then  is a child of  along branch 1.
In all cases  is a "smaller" coprime pair with  This process of "computing the father" can stop only if either  or  In these cases, coprimality, implies that the pair is either  or

Applications 

In machine design, an even, uniform gear wear is achieved by choosing the tooth counts of the two gears meshing together to be relatively prime. When a 1:1 gear ratio is desired, a gear relatively prime to the two equal-size gears may be inserted between them.

In pre-computer cryptography, some Vernam cipher machines combined several loops of key tape of different lengths. Many rotor machines combine rotors of different numbers of teeth. Such combinations work best when the entire set of lengths are pairwise coprime.

Generalizations 
This concept can be extended to other algebraic structures than  for example, polynomials whose greatest common divisor is 1 are called coprime polynomials.

See also

Euclid's orchard
Superpartient number

Notes

References

Further reading
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Number theory