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lcm(m, n) (least common multiple of m and n) is the product of all prime factors of m or n (with the largest multiplicity for m or n). gcd(m, n) × lcm(m, n) = m × n. Finding the prime factors is often harder than computing gcd and lcm using other algorithms which do not require known prime factorization.
Continuing this process until every factor is prime is called prime factorization; the result is always unique up to the order of the factors by the prime factorization theorem. To factorize a small integer n using mental or pen-and-paper arithmetic, the simplest method is trial division : checking if the number is divisible by prime numbers 2 ...
The requirement that the factors be prime is necessary: factorizations containing composite numbers may not be unique (for example, = =). This theorem is one of the main reasons why 1 is not considered a prime number : if 1 were prime, then factorization into primes would not be unique; for example, 2 = 2 ⋅ 1 = 2 ⋅ 1 ⋅ 1 ...
For each odd prime power the corresponding factor (/) is the cyclic group of order () =, which may further factor into cyclic groups of prime-power orders. For powers of 2 the factor ( Z / 2 k Z ) × {\displaystyle (\mathbb {Z} /{2^{k}}\mathbb {Z} )^{\times }} is not cyclic unless k = 0, 1, 2, but factors into cyclic groups as described above.
Another proof, by the Swiss mathematician Leonhard Euler, relies on the fundamental theorem of arithmetic: that every integer has a unique prime factorization.What Euler wrote (not with this modern notation and, unlike modern standards, not restricting the arguments in sums and products to any finite sets of integers) is equivalent to the statement that we have [9]
Because the prime factorization of a highly composite number uses all of the first k primes, every highly composite number must be a practical number. [8] Due to their ease of use in calculations involving fractions , many of these numbers are used in traditional systems of measurement and engineering designs.
Fermat's factorization method, named after Pierre de Fermat, is based on the representation of an odd integer as the difference of two squares: N = a 2 − b 2 . {\displaystyle N=a^{2}-b^{2}.} That difference is algebraically factorable as ( a + b ) ( a − b ) {\displaystyle (a+b)(a-b)} ; if neither factor equals one, it is a proper ...
For prime powers, efficient classical factorization algorithms exist, [22] hence the rest of the quantum algorithm may assume that is not a prime power. If those easy cases do not produce a nontrivial factor of N {\displaystyle N} , the algorithm proceeds to handle the remaining case.