# Coprime

Coprime

In number theory, a branch of mathematics, two integers a and b are said to be coprime (also spelled co-prime) or relatively prime if the only positive integer that evenly divides both of them is 1. This is the same thing as their greatest common divisor being 1. In addition to $\gcd(a, b) = 1\;$ and $(a, b) = 1,\;$ the notation a $\perp$  b is sometimes used to indicate that a and b are relatively prime.

For example, 14 and 15 are coprime, being commonly divisible by only 1, but 14 and 21 are not, because they are both divisible by 7. The numbers 1 and −1 are coprime to every integer, and they are the only integers to be coprime with 0.

A fast way to determine whether two numbers are coprime is given by the Euclidean algorithm.

The number of integers coprime to a positive integer n, between 1 and n, is given by Euler's totient function (or Euler's phi function) φ(n).

## Properties  Figure 1. The numbers 4 and 9 are coprime. Therefore, the diagonal of a 4 x 9 lattice does not intersect any other lattice points

There are a number of conditions which are equivalent to a and b being coprime:

As a consequence of the third point, if a and b are coprime and brbs (mod a), then rs (mod a) (because we may "divide by b" when working modulo a). Furthermore, if b1 and b2 are both coprime with a, then so is their product b1b2 (modulo a 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 p divides a product bc, then p divides at least one of the factors b, c.

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

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

The two integers a and b are coprime if and only if the point with coordinates (a, b) in a Cartesian coordinate system is "visible" from the origin (0,0), in the sense that there is no point with integer coordinates between the origin and (a, b). (See figure 1.)

In a sense that can be made precise, the probability that two randomly chosen integers are coprime is 6/π2 (see pi), which is about 61%. See below.

Two natural numbers a and b are coprime if and only if the numbers 2a − 1 and 2b − 1 are coprime. As a generalization of this, following easily from Euclidean algorithm in base n > 1:

gcd(na − 1,nb − 1) = ngcd(a,b) − 1.

## Cross notation, group

If n≥1 and is an integer, the numbers coprime to n, taken modulo n, form a group with multiplication as operation; it is written as (Z/nZ)× or Zn*.

## Generalizations

Two ideals A and B in the commutative ring R are called coprime (or comaximal) if A + B = R. This generalizes Bézout's identity: with this definition, two principal ideals (a) and (b) in the ring of integers Z are coprime if and only if a and b are coprime.

If the ideals A and B of R are coprime, then AB = AB; furthermore, if C is a third ideal such that A contains BC, then A contains C. The Chinese remainder theorem is an important statement about coprime ideals.

The concept of being relatively prime can also be extended to any finite set of integers S = {a1, a2, .... an} to mean that the greatest common divisor of the elements of the set is 1. If every pair in a (finite or infinite) set of integers is relatively prime, then the set is called pairwise relatively prime.

Every pairwise relatively prime finite set is relatively prime; however, the converse is not true: {6, 10, 15} is relatively prime, but not pairwise relative prime (in fact, each pair of integers in the set has a non-trivial common factor).

## Probabilities

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

Intuitively, the probability that any number is divisible by a prime (or any integer), p is 1 / p (for example, every 7th integer is divisible by 7.) Hence the probability that two numbers are both divisible by this prime is 1 / p2, and the probability that at least one of them is not is 1 − 1 / p2. Now, for distinct primes, these divisibility events are mutually independent. (This would not, in general, be true if they were not prime.) (For the case of two events, a number is divisible by p and q if and only if it is divisible by pq; the latter has probability 1/pq.) Thus the probability that two numbers are coprime is given by a product over all primes, $\prod_p^{\infty} \left(1-\frac{1}{p^2}\right) = \left( \prod_p^{\infty} \frac{1}{1-p^{-2}} \right)^{-1} = \frac{1}{\zeta(2)} = \frac{6}{\pi^2} \approx 0.607927102 \approx 61%.$

Here ζ refers to the Riemann zeta function, the identity relating the product over primes to ζ(2) is an example of an Euler product, and the evaluation of ζ(2) as π2/6 is the Basel problem, solved by Leonhard Euler in 1735. In general, the probability of k randomly chosen integers being coprime is 1/ζ(k).

There is often confusion about what a "randomly chosen integer" is. One way of understanding this is to assume that the integers are chosen randomly between 1 and an integer N. Then, for each upper bound N, there is a probability PN that two randomly chosen numbers are coprime. This will never be exactly 6 / π2, but in the limit as $N \to \infty$, $P_N \to 6/\pi^2$.

## Generating all coprime pairs

All pairs of coprime numbers m,n can be generated in a parent-3 children-9 grandchildren... family tree starting from (2,1) (for even-odd or odd-even pairs) or from (3,1) (for odd-odd pairs), with three branches from each node. The branches are generated as follows:

Branch 1: mnext = 2mn, and nnext = m

Branch 2: mnext = 2m + n, and nnext = m

Branch 3: mnext = m + 2n, and nnext = n

This family tree is exhaustive and non-redundant with no invalid members.

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