In arithmetic, the Euclidean division is the conventional process of division of two integers producing a quotient and a remainder. There is a theorem stating that the quotient and remainder exist, are unique, and have certain properties. Integer division algorithms compute the quotient and remainder given two integers, the most well-known such algorithm being long division. The integer division algorithm is an important ingredient for other algorithms, such as the Euclidean algorithm for finding the greatest common divisor of two integers.
Intuitive example 
Suppose that a pie has 9 slices and they are to be divided evenly among 4 people. Using Euclidean division, 9 divided by 4 is 2 with remainder 1. In other words, each person receives 2 slices of pie, and there is 1 slice left over.
This can be confirmed using multiplication, the inverse of division: if each of the 4 people received 2 slices, then 4 × 2 = 8 slices were given out in all. Adding the 1 slice remaining, the result is 9 slices. In summary: 9 = 4 × 2 + 1.
In general, if the number of slices is denoted a and the number of people is b, one can divide the pie evenly among the people such that each person receives q slices (the quotient) and some number of slices r < b are left over (the remainder). Regardless, the equation a = bq + r holds.
If 9 slices were divided among 3 people instead of 4, each would receive 3 and no slices would be left over. In this case the remainder is zero, and it is said that 3 evenly divides 9, or that 3 divides 9.
Euclidean division can also be extended to negative integers using the same formula; for example −9 = 4 × (−3) + 3, so −9 divided by 4 is −3 with remainder 3. The remainder is the only one of the four numbers that can never be negative.
Statement of the theorem 
The four integers that appear in this theorem have been given a name: a is called the dividend, b is called the divisor, q is called the quotient and r is called the remainder.
The computation of the quotient and the remainder from the dividend and the divisor is called division or, in case of ambiguity, Euclidean division. The theorem is frequently referred to as the division algorithm, although it is a theorem and not an algorithm, because its proof as given below also provides a simple division algorithm for computing q and r.
Division is not defined in the case where b = 0; see division by zero.
- If a = 7 and b = 3, then q = 2 and r = 1, since 7 = 3 × 2 + 1.
- If a = 7 and b = −3, then q = −2 and r = 1, since 7 = −3 × (−2) + 1.
- If a = −7 and b = 3, then q = −3 and r = 2, since −7 = 3 × (−3) + 2.
- If a = −7 and b = −3, then q = 3 and r = 2, since −7 = −3 × 3 + 2.
The proof consists of two parts — first, the proof of the existence of q and r, and second, the proof of the uniqueness of q and r.
Consider first the case b < 0. Setting b' = −b and q' = −q, the equation a = bq + r may be rewritten a = b'q' + r and the inequality 0 ≤ r < |b| may be rewritten 0 ≤ r < |b'|. This reduces the existence for the case b < 0 to that of the case b > 0.
Similarly, if a < 0 and b > 0, setting a' = −a, q' = −q − 1 and r' = b − r, the equation a = bq + r may be rewritten a' = bq' + r' and the inequality 0 ≤ r < b may be rewritten 0 ≤ r' < b. Thus the proof of the existence is reduced to the case a ≥ 0 and b > 0 and we consider only this case in the remainder of the proof.
Let q1 and r1, both nonnegative, such that a = bq1 + r1, for example q1 = 0 and r1 = a. If r1 < b, we are done. Otherwise q2 = q1 + 1 and r2 = r1 − b satisfy a = bq2 + r2 and 0 < r2 < r1. Repeating this process one gets eventually q = qk and r = rk such that a = bq + r and 0 ≤ r < b.
This proves the existence and also gives a simple division algorithm to compute the quotient and the remainder. However this algorithm needs q steps and is thus not efficient.
Suppose there exists q, q' , r, r' with 0 ≤ r, r' < |b| such that a = bq + r and a = bq' + r' . Adding the two inequalities 0 ≤ r < |b| and −|b| < −r' ≤ 0 yields −|b| < r − r' < |b|, that is |r − r' | < |b|.
Subtracting the two equations yields: b(q' − q) = (r − r' ). Thus |b| divides |r − r' |. If |r − r' | ≠ 0 this implies |b| < |r − r' |, contradicting previous inequality. Thus, r = r' and b(q' − q) = 0. As b ≠ 0, this implies q = q' , proving uniqueness.
Other proofs 
Usually, an existence proof does not provide an algorithm to compute the existing object, but the above proof provides immediately an algorithm (see Division algorithm#Division_by_repeated_subtraction). However this is not a very efficient method, as it requires as many steps as the size of the quotient. This is related to the fact that it only uses addition, subtraction and comparison of the integers, without involving multiplication, nor any particular representation of the integers, such as decimal notation.
In terms of decimal notation, long division provides a much more efficient division algorithm. Its generalization to binary notation allows to use it in a computer. However, for large inputs, algorithms that reduce division to multiplication, like Newton–Raphson one, are usually preferred, because they need a time which is proportional to the time of the multiplication needed to verify the result, independently of the multiplication algorithm which is used.
In domains other than integers 
Euclidean domains are defined as integral domains which support the following generalization of Euclidean division: Given an element a and a non-zero element b in a Euclidean domain R equipped with a Euclidean function d, there exist q and r in R such that a = bq + r and either r = 0 or d(r) < d(b). Unlike in the integer case, q and r need not be unique.
Examples of Euclidean domains include fields, polynomial rings in one variable over a field, and the Gaussian integers. The Euclidean division of polynomials has been the object of specific developments. See Polynomial long division, Polynomial greatest common divisor#Euclidean division and Polynomial greatest common divisor#Pseudo-remainder sequences.
Generalized division algorithms 
The 1st generalized division algorithm: Given integers , , with , there exist unique integers and with such that .
Especially, if then . In this case, is called the least absolute remainder. As an application of this generalization, the original Euclidean algorithm for integers can be slightly sped up.
The 2nd generalized division algorithm: Given integers , with , and let be the multiplicative inverse of . Then there exist unique integers and with such that .
This result generalizes Hensel's odd division (1900), and its proof can be found in.
The value in the 2nd generalization corresponds to the N-residue defined in Montgomery reduction.
- Burton, David M. (2010). Elementary Number Theory. McGraw-Hill. pp. 17–19. ISBN 978-0-07-338314-9.
- Durbin, John R. (1992). Modern Algebra : an Introduction (3rd ed. ed.). New York: Wiley. p. 63. ISBN 0-471-51001-7.
- Haining Fan, Ming Gu, Jiaguang Sun and Kwok-Yan Lam: "Obtaining More Karatsuba-Like Formulae over the Binary Field", 2012