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Fermat's method works best when there is a factor near the square-root of N. If the approximate ratio of two factors ( d / c {\displaystyle d/c} ) is known, then a rational number v / u {\displaystyle v/u} can be picked near that value.
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, 3, 5, and so on, up to the square root of n. For larger numbers, especially when using a computer, various more sophisticated factorization algorithms are more efficient.
Hensel's original lemma concerns the relation between polynomial factorization over the integers and over the integers modulo a prime number p and its powers. It can be straightforwardly extended to the case where the integers are replaced by any commutative ring, and p is replaced by any maximal ideal (indeed, the maximal ideals of have the form , where p is a prime number).
The size of the input to the algorithm is log 2 n or the number of bits in the binary representation of n. Any element of the order n c for a constant c is exponential in log n. The running time of the number field sieve is super-polynomial but sub-exponential in the size of the input.
Note that this algorithm may fail to find a nontrivial factor even when n is composite. In that case, the method can be tried again, using a starting value of x other than 2 ( 0 ≤ x < n {\displaystyle 0\leq x<n} ) or a different g ( x ) {\displaystyle g(x)} , g ( x ) = ( x 2 + b ) mod n {\displaystyle g(x)=(x^{2}+b){\bmod {n}}} , with ...
Then the last prime number worth testing as a possible factor of n is P i where P 2 i + 1 > n; equality here would mean that P i + 1 is a factor. Thus, testing with 2, 3, and 5 suffices up to n = 48 not just 25 because the square of the next prime is 49, and below n = 25 just 2 and 3 are sufficient. Should the square root of n be an integer ...
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.
A natural number is a sociable Dudeney root if it is a periodic point for ,, where , = for a positive integer , and forms a cycle of period . A Dudeney root is a sociable Dudeney root with k = 1 {\displaystyle k=1} , and a amicable Dudeney root is a sociable Dudeney root with k = 2 {\displaystyle k=2} .