Bézout's identity — Let a and b be integers or polynomials with greatest common divisor d. Then there exist integers or polynomials x and y such that ax + by = d. Moreover, the integers or polynomials of the form az + bt are exactly the multiples of d.
Here the greatest common divisor of 0 and 0 is taken to be 0. The integers or polynomials x and y are called Bézout coefficients for (a, b); they are not unique. A pair of Bézout coefficients can be computed by the extended Euclidean algorithm, and this pair is, in the case of integers one of the two pairs such that and equality occurs only if one of a and b is a multiple of the other. In the polynomial case, the extended Euclidean algorithm produces the unique pair such that or (both inequalities are verified except one of a and b is a multiple of the other).
As an example, the greatest common divisor of 15 and 69 is 3, and 3 can be written as a combination of 15 and 69 as 3 = 15 × (−9) + 69 × 2, with Bézout coefficients −9 and 2.
A Bézout domain is an integral domain in which Bézout's identity holds. In particular, Bézout's identity holds in principal ideal domains. Every theorem that results from Bézout's identity is thus true in all principal ideal domains.
Structure of solutions
If a and b are not both zero and one pair of Bézout coefficients (x, y) has been computed (e.g., using extended Euclidean algorithm), all pairs can be represented in the form
where k is an arbitrary integer, d is the greatest common divisor of a and b, and the fractions simplify to integers.
If a and b are both nonzero, then exactly two of these pairs of Bézout coefficients satisfy
This relies on a property of Euclidean division: given two non-zero integers c and d, if d does not divide c, there is exactly one pair (q, r) such that c = dq + r and 0 < r < |d|, and another one such that c = dq + r and −|d| < r < 0.
The two pairs of small Bézout's coefficients are obtained from the given one (x, y) by choosing for k in the above formula either of the two integers next to .
The extended Euclidean algorithm always produces one of these two minimal pairs.
Let a = 12 and b = 42, then gcd (12, 42) = 6. Then the following Bézout's identities are had, with the Bézout coefficients written in red for the minimal pairs and in blue for the other ones.
If (x, y) = (18, −5) is the original pair of Bézout coefficients, then yields the minimal pairs via k = 2, respectively k = 3; that is, (18 − 2 ⋅ 7, −5 + 2 ⋅ 2) = (4, −1), and (18 − 3 ⋅ 7, −5 + 3 ⋅ 2) = (−3, 1).
Given any nonzero integers a and b, let The set S is nonempty since it contains either a or –a (with x = ±1 and y = 0). Since S is a nonempty set of positive integers, it has a minimum element . To prove that d is the greatest common divisor of a and b, it must be proven that d is a common divisor of a and b, and that for any other common divisor c, one has c ≤ d.
The Euclidean division of a by d may be written
Now, let c be any common divisor of a and b; that is, there exist u and v such that a = cu and b = cv. One has thus
For three or more integers
Bézout's identity can be extended to more than two integers: if
- d is the smallest positive integer of this form
- every number of this form is a multiple of d
Bézout's identity works for univariate polynomials over a field exactly in the same ways as for integers. In particular the Bézout's coefficients and the greatest common divisor may be computed with the extended Euclidean algorithm.
The generalization of this result to any number of polynomials and indeterminates is Hilbert's Nullstellensatz.
For principal ideal domains
As noted in the introduction, Bézout's identity works not only in the ring of integers, but also in any other principal ideal domain (PID). That is, if R is a PID, and a and b are elements of R, and d is a greatest common divisor of a and b, then there are elements x and y in R such that ax + by = d. The reason is that the ideal Ra + Rb is principal and equal to Rd.
An integral domain in which Bézout's identity holds is called a Bézout domain.
French mathematician Étienne Bézout (1730–1783) proved this identity for polynomials. This statement for integers can be found already in the work of an earlier French mathematician, Claude Gaspard Bachet de Méziriac (1581–1638).
- AF+BG theorem, an analogue of Bézout's identity for homogeneous polynomials in three indeterminates
- Fundamental theorem of arithmetic
- Euclid's lemma
- Bézout, É. (1779). Théorie générale des équations algébriques. Paris, France: Ph.-D. Pierres.
- Tignol, Jean-Pierre (2001). Galois' Theory of Algebraic Equations. Singapore: World Scientific. ISBN 981-02-4541-6.
- Claude Gaspard Bachet (sieur de Méziriac) (1624). Problèmes plaisants & délectables qui se font par les nombres (2nd ed.). Lyons, France: Pierre Rigaud & Associates. pp. 18–33. On these pages, Bachet proves (without equations) "Proposition XVIII. Deux nombres premiers entre eux estant donnez, treuver le moindre multiple de chascun d’iceux, surpassant de l’unité un multiple de l’autre." (Given two numbers [which are] relatively prime, find the lowest multiple of each of them [such that] one multiple exceeds the other by unity (1).) This problem (namely, ax - by = 1) is a special case of Bézout's equation and was used by Bachet to solve the problems appearing on pages 199 ff.
- See also: Maarten Bullynck (February 2009). "Modular arithmetic before C.F. Gauss: Systematizations and discussions on remainder problems in 18th-century Germany" (PDF). Historia Mathematica. 36 (1): 48–72. doi:10.1016/j.hm.2008.08.009.