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In number theory, two integers $a$ and $b$ are said to be relatively prime, mutually prime, or coprime (also written co-prime) if the only positive integer (factor) that divides both of them is 1. Consequently, any prime number that divides one does not divide the other. This is equivalent to their greatest common divisor (gcd) being 1.

The numerator and denominator of a reduced fraction are coprime. As specific examples, 14 and 15 are coprime, being commonly divisible only by 1, while 14 and 21 are not coprime, because they are both divisible by 7.

Standard notations for relatively prime integers $a$ and $b$ are: $gcd(a, b) = 1$ and $(a, b) = 1$. Graham, Knuth and Patashnik have proposed that the notation $$a\perp b$$ be used to indicate that $a$ and $b$ are relatively prime and that the term "prime" be used instead of coprime (as in $a$ is prime to $b$).

A fast way to determine whether two numbers are coprime is given by the Euclidean algorithm. The Euclidean algorithm takes two numbers and finds their greatest common divisor by taking the smaller number and the remainder of the larger number divided by the smaller number until the remainder is zero. If the last number is one, then the two original numbers are coprime. There are several proofs of this concept which may be accessed on the Euclidean algorithm page.

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) $φ(n)$.

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 $a$ and $b$ are coprime for every pair $(a, b)$ of different integers in the set. The set ${2, 3, 4}$ is coprime, but it is not pairwise coprime since 2 and 4 are not relatively prime.