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In mathematics, the Euclidean algorithm, or Euclid's algorithm, is an efficient method for computing the greatest common divisor (GCD) of two numbers, the largest number that divides both of them without leaving a remainder. It is named after the ancient Greek mathematician Euclid, who first described it in Euclid's ''Elements'' (c. 300 BC). It is an example of an ''algorithm'', a step-by-step procedure for performing a calculation according to well-defined rules, and is one of the oldest numerical algorithms in common use. It can be used to reduce fractions to their simplest form, and is a part of many other number-theoretic and cryptographic calculations. The Euclidean algorithm is based on the principle that the greatest common divisor of two numbers does not change if the larger number is replaced by its difference with the smaller number. For example, 21 is the GCD of 252 and 105 (252 = 21 × 12 and 105 = 21 × 5), and the same number 21 is also the GCD of 105 and 147 = 252 − 105. Since this replacement reduces the larger of the two numbers, repeating this process gives successively smaller pairs of numbers until one of the two numbers reaches zero. When that occurs, the other number (the one that is not zero) is the GCD of the original two numbers. By reversing the steps, the GCD can be expressed as a sum of the two original numbers each multiplied by a positive or negative integer, e.g., 21 = 5 × 105 + (−2) × 252. The fact that the GCD can always be expressed in this way is known as Bézout's identity. The version of the Euclidean algorithm described above (and by Euclid) can take many subtraction steps to find the GCD when one of the given numbers is much bigger than the other. A more efficient version of the algorithm shortcuts these steps, instead replacing the larger of the two numbers by its remainder when divided by the smaller of the two. With this improvement, the algorithm never requires more steps than five times the number of digits (base 10) of the smaller integer. This was proven by Gabriel Lamé in 1844, and marks the beginning of computational complexity theory. Additional methods for improving the algorithm's efficiency were developed in the 20th century. The Euclidean algorithm has many theoretical and practical applications. It is used for reducing fractions to their simplest form and for performing division in modular arithmetic. Computations using this algorithm form part of the cryptographic protocols that are used to secure internet communications, and in methods for breaking these cryptosystems by factoring large composite numbers. The Euclidean algorithm may be used to solve Diophantine equations, such as finding numbers that satisfy multiple congruences according to the Chinese remainder theorem, to construct continued fractions, and to find accurate rational approximations to real numbers. Finally, it is a basic tool for proving theorems in number theory such as Lagrange's four-square theorem and the uniqueness of prime factorizations. The original algorithm was described only for natural numbers and geometric lengths (real numbers), but the algorithm was generalized in the 19th century to other types of numbers, such as Gaussian integers and polynomials of one variable. This led to modern abstract algebraic notions such as Euclidean domains. ==Background: greatest common divisor== (詳細はnatural numbers ''a'' and ''b''. The greatest common divisor ''g'' is the largest natural number that divides both ''a'' and ''b'' without leaving a remainder. Synonyms for the GCD include the ''greatest common factor'' (GCF), the ''highest common factor'' (HCF), and the ''greatest common measure'' (GCM). The greatest common divisor is often written as gcd(''a'', ''b'') or, more simply, as (''a'', ''b''), although the latter notation is also used for other mathematical concepts, such as two-dimensional vectors. If gcd(''a'', ''b'') = 1, then ''a'' and ''b'' are said to be coprime (or relatively prime). This property does not imply that ''a'' or ''b'' are themselves prime numbers. For example, neither 6 nor 35 is a prime number, since they both have two prime factors: 6 = 2 × 3 and 35 = 5 × 7. Nevertheless, 6 and 35 are coprime. No natural number other than 1 divides both 6 and 35, since they have no prime factors in common. Let ''g'' = gcd(''a'', ''b''). Since ''a'' and ''b'' are both multiples of ''g'', they can be written ''a'' = ''mg'' and ''b'' = ''ng'', and there is no larger number ''G'' > ''g'' for which this is true. The natural numbers ''m'' and ''n'' must be coprime, since any common factor could be factored out of ''m'' and ''n'' to make ''g'' greater. Thus, any other number ''c'' that divides both ''a'' and ''b'' must also divide ''g''. The greatest common divisor ''g'' of ''a'' and ''b'' is the unique (positive) common divisor of ''a'' and ''b'' that is divisible by any other common divisor ''c''. The GCD can be visualized as follows. Consider a rectangular area ''a'' by ''b'', and any common divisor ''c'' that divides both ''a'' and ''b'' exactly. The sides of the rectangle can be divided into segments of length ''c'', which divides the rectangle into a grid of squares of side length ''c''. The greatest common divisor ''g'' is the largest value of ''c'' for which this is possible. For illustration, 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 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 GCD of two numbers ''a'' and ''b'' is the product of the prime factors shared by the two numbers, where a same prime factor can be used multiple times, but only as long as the product of these factors divides both ''a'' and ''b''. For example, since 1386 can be factored into 2 × 3 × 3 × 7 × 11, and 3213 can be factored into 3 × 3 × 3 × 7 × 17, the greatest common divisor of 1386 and 3213 equals 63 = 3 × 3 × 7, the product of their shared prime factors. If two numbers have no prime factors in common, their greatest common divisor is 1 (obtained here as an instance of the empty product), in other words they are coprime. A key advantage of the Euclidean algorithm is that it can find the GCD efficiently without having to compute the prime factors. Factorization of large integers is believed to be a computationally very difficult problem, and the security of many modern cryptography systems is based upon its infeasibility. Another definition of the GCD is helpful in advanced mathematics, particularly ring theory.〔 The greatest common divisor ''g'' of two nonzero numbers ''a'' and ''b'' is also their smallest positive integral linear combination, that is, the smallest positive number of the form ''ua'' + ''vb'' where ''u'' and ''v'' are integers. The set of all integral linear combinations of ''a'' and ''b'' is actually the same as the set of all multiples of ''g'' (''mg'', where ''m'' is an integer). In modern mathematical language, the ideal generated by ''a'' and ''b'' is the ideal generated by ''g'' alone (an ideal generated by a single element is called a principal ideal, and all ideals of the integers are principal ideals). Some properties of the GCD are in fact easier to see with this description, for instance the fact that any common divisor of ''a'' and ''b'' also divides the GCD (it divides both terms of ''ua'' + ''vb''). The equivalence of this GCD definition with the other definitions is described below. The GCD of three or more numbers equals the product of the prime factors common to all the numbers, but it can also be calculated by repeatedly taking the GCDs of pairs of numbers. For example, : Thus, Euclid's algorithm, which computes the GCD of two integers, suffices to calculate the GCD of arbitrarily many integers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Euclidean algorithm」の詳細全文を読む スポンサード リンク
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