# Greatest common divisor

In the name "greatest common divisor", the adjective "greatest" may be replaced by "highest", and the word "divisor" may be replaced by "factor", so that other names include **highest common factor** (**hcf**), etc.^{[3]}^{[4]}^{[5]}^{[6]} Historically, other names for the same concept have included **greatest common measure**.^{[7]}

This notion can be extended to polynomials (see Polynomial greatest common divisor) and other commutative rings (see § In commutative rings below).

The *greatest common divisor* (GCD) of two nonzero integers a and b is the greatest positive integer d such that d is a divisor of both a and b; that is, there are integers e and f such that *a* = *de* and *b* = *df*, and d is the largest such integer. The GCD of a and b is generally denoted gcd(*a*, *b*).^{[8]}

This definition also applies when one of a and b is zero. In this case, the GCD is the absolute value of the non zero integer: gcd(*a*, 0) = gcd(0, *a*) = |*a*|. This case is important as the terminating step of the Euclidean algorithm.

The GCD of a and b is their greatest positive common divisor in the preorder relation of divisibility. This means that the common divisors of a and b are exactly the divisors of their GCD. This is commonly proved by using either Euclid's lemma, the fundamental theorem of arithmetic, or the Euclidean algorithm. This is the meaning of "greatest" that is used for the generalizations of the concept of GCD.

The number 54 can be expressed as a product of two integers in several different ways:

Computing all divisors of the two numbers in this way is usually not efficient, especially for large numbers that have many divisors. Much more efficient methods are described in § Calculation.

Two numbers are called relatively prime, or coprime, if their greatest common divisor equals 1.^{[14]} For example, 9 and 28 are Coprime.

For example, a 24-by-60 rectangular area can be divided into a grid of: 1-by-1 squares, 2-by-2 squares, 3-by-3 squares, 4-by-4 squares, 6-by-6 squares or 12-by-12 squares. Therefore, 12 is the greatest common divisor of 24 and 60. A 24-by-60 rectangular area can thus be divided into a grid of 12-by-12 squares, with two squares along one edge (24/12 = 2) and five squares along the other (60/12 = 5).

The greatest common divisor is useful for reducing fractions to the lowest terms.^{[15]} For example, gcd(42, 56) = 14, therefore,

The greatest common divisor can be used to find the least common multiple of two numbers when the greatest common divisor is known, using the relation,

Greatest common divisors can be computed by determining the prime factorizations of the two numbers and comparing factors. For example, to compute gcd(48, 180), we find the prime factorizations 48 = 2^{4} · 3^{1} and 180 = 2^{2} · 3^{2} · 5^{1}; the GCD is then 2^{min(4,2)} · 3^{min(1,2)} · 5^{min(0,1)} = 2^{2} · 3^{1} · 5^{0} = 12, as shown in the Venn diagram. The corresponding LCM is then 2^{max(4,2)} · 3^{max(1,2)} · 5^{max(0,1)} = 2^{4} · 3^{2} · 5^{1} = 720.

In practice, this method is only feasible for small numbers, as computing prime factorizations takes too long.

The method introduced by Euclid for computing greatest common divisors is based on the fact that, given two positive integers a and b such that *a* > *b*, the common divisors of a and b are the same as the common divisors of *a* – *b* and b.

So, Euclid's method for computing the greatest common divisor of two positive integers consists of replacing the larger number by the difference of the numbers, and repeating this until the two numbers are equal: that is their greatest common divisor.

This method can be very slow if one number is much larger than the other. So, the variant that follows is generally preferred.

A more efficient method is the *Euclidean algorithm*, a variant in which the difference of the two numbers a and b is replaced by the *remainder* of the Euclidean division (also called *division with remainder*) of a by b.

Denoting this remainder as *a* mod *b*, the algorithm replaces (*a*, *b*) by (*b*, *a* mod *b*) repeatedly until the pair is (*d*, 0), where d is the greatest common divisor.

Lehmer's algorithm is based on the observation that the initial quotients produced by Euclid's algorithm can be determined based on only the first few digits; this is useful for numbers that are larger than a computer word. In essence, one extracts initial digits, typically forming one or two computer words, and runs Euclid's algorithms on these smaller numbers, as long as it is guaranteed that the quotients are the same with those that would be obtained with the original numbers. Those quotients are collected into a small 2-by-2 transformation matrix (that is a matrix of single-word integers), for using them all at once for reducing the original numbers^{[clarification needed]}. This process is repeated until numbers are small enough that the binary algorithm (see below) is more efficient.

This algorithm improves speed, because it reduces the number of operations on very large numbers, and can use hardware arithmetic for most operations. In fact, most of the quotients are very small, so a fair number of steps of the Euclidean algorithm can be collected in a 2-by-2 matrix of single-word integers. When Lehmer's algorithm encounters a quotient that is too large, it must fall back to one iteration of Euclidean algorithm, with a Euclidean division of large numbers.

The binary GCD algorithm uses only subtraction and division by 2.
The method is as follows: Let *a* and *b* be the two non-negative integers. Let the integer *d* be 0. There are five possibilities:

As gcd(*a*, *a*) = *a*, the desired GCD is *a* × 2^{d} (as *a* and *b* are changed in the other cases, and *d* records the number of times that *a* and *b* have been both divided by 2 in the next step, the GCD of the initial pair is the product of *a* and 2^{d}).

Then 2 is a common divisor. Divide both *a* and *b* by 2, increment *d* by 1 to record the number of times 2 is a common divisor and continue.

As gcd(*a*,*b*) = gcd(*b*,*a*), if *a* < *b* then exchange *a* and *b*. The number *c* = *a* − *b* is positive and smaller than *a*. Any number that divides *a* and *b* must also divide *c* so every common divisor of *a* and *b* is also a common divisor of *b* and *c*. Similarly, *a* = *b* + *c* and every common divisor of *b* and *c* is also a common divisor of *a* and *b*. So the two pairs (*a*, *b*) and (*b*, *c*) have the same common divisors, and thus gcd(*a*,*b*) = gcd(*b*,*c*). Moreover, as *a* and *b* are both odd, *c* is even, the process can be continued with the pair (*a*, *b*) replaced by the smaller numbers (*c*/2, *b*) without changing the GCD.

Each of the above steps reduces at least one of *a* and *b* while leaving them non-negative and so can only be repeated a finite number of times. Thus eventually the process results in *a* = *b*, the stopping case. Then the GCD is *a* × 2^{d}.

Example: (*a*, *b*, *d*) = (48, 18, 0) → (24, 9, 1) → (12, 9, 1) → (6, 9, 1) → (3, 9, 1) → (3, 3, 1) ; the original GCD is thus the product 6 of 2^{d} = 2^{1} and *a*= *b*= 3.

The binary GCD algorithm is particularly easy to implement on binary computers. Its computational complexity is

If *a* and *b* are both nonzero, the greatest common divisor of *a* and *b* can be computed by using least common multiple (LCM) of *a* and *b*:

which generalizes to *a* and *b* rational numbers or commensurable real numbers.

which is a function that can be evaluated for complex *b*.^{[17]} Wolfgang Schramm has shown that

is an entire function in the variable *b* for all positive integers *a* where *c*_{d}(*k*) is Ramanujan's sum.^{[18]}

Previous complexities are valid for the usual models of computation, specifically multitape Turing machines and random-access machines.

The computation of the greatest common divisors belongs thus to the class of problems solvable in quasilinear time. *A fortiori*, the corresponding decision problem belongs to the class P of problems solvable in polynomial time. The GCD problem is not known to be in NC, and so there is no known way to parallelize it efficiently; nor is it known to be P-complete, which would imply that it is unlikely to be possible to efficiently parallelize GCD computation. Shallcross et al. showed that a related problem (EUGCD, determining the remainder sequence arising during the Euclidean algorithm) is NC-equivalent to the problem of integer linear programming with two variables; if either problem is in **NC** or is **P-complete**, the other is as well.^{[20]} Since **NC** contains NL, it is also unknown whether a space-efficient algorithm for computing the GCD exists, even for nondeterministic Turing machines.

Using this information, the expected value of the greatest common divisor function can be seen (informally) to not exist when *k* = 2. In this case the probability that the GCD equals *d* is *d*^{−2}/ζ(2), and since ζ(2) = π^{2}/6 we have

This last summation is the harmonic series, which diverges. However, when *k* ≥ 3, the expected value is well-defined, and by the above argument, it is

For *k* = 3, this is approximately equal to 1.3684. For *k* = 4, it is approximately 1.1106.

The notion of greatest common divisor can more generally be defined for elements of an arbitrary commutative ring, although in general there need not exist one for every pair of elements.

If R is a commutative ring, and a and b are in R, then an element d of R is called a *common divisor* of a and b if it divides both a and b (that is, if there are elements x and y in R such that *d*·*x* = *a* and *d*·*y* = *b*).
If d is a common divisor of a and b, and every common divisor of a and b divides d, then d is called a *greatest common divisor* of a and *b*.

With this definition, two elements a and b may very well have several greatest common divisors, or none at all. If R is an integral domain then any two GCD's of a and b must be associate elements, since by definition either one must divide the other; indeed if a GCD exists, any one of its associates is a GCD as well. Existence of a GCD is not assured in arbitrary integral domains. However, if R is a unique factorization domain, then any two elements have a GCD, and more generally this is true in GCD domains.
If R is a Euclidean domain in which euclidean division is given algorithmically (as is the case for instance when *R* = *F*[*X*] where F is a field, or when R is the ring of Gaussian integers), then greatest common divisors can be computed using a form of the Euclidean algorithm based on the division procedure.

The following is an example of an integral domain with two elements that do not have a GCD:

The elements 2 and 1 + √−3 are two maximal common divisors (that is, any common divisor which is a multiple of 2 is associated to 2, the same holds for 1 + √−3, but they are not associated, so there is no greatest common divisor of a and *b*.

Corresponding to the Bézout property we may, in any commutative ring, consider the collection of elements of the form *pa* + *qb*, where p and q range over the ring. This is the ideal generated by a and b, and is denoted simply (*a*, *b*). In a ring all of whose ideals are principal (a principal ideal domain or PID), this ideal will be identical with the set of multiples of some ring element *d*; then this d is a greatest common divisor of a and *b*. But the ideal (*a*, *b*) can be useful even when there is no greatest common divisor of a and *b*. (Indeed, Ernst Kummer used this ideal as a replacement for a GCD in his treatment of Fermat's Last Theorem, although he envisioned it as the set of multiples of some hypothetical, or *ideal*, ring element d, whence the ring-theoretic term.)