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factor base : ウィキペディア英語版
factor base
In computational number theory, a factor base is a small set of prime numbers commonly used as a mathematical tool in algorithms involving extensive sieving for potential factors of a given integer.
==Usage in factoring algorithms==
A factor base is a relatively small set of distinct prime numbers ''P'', sometimes together with -1. Say we want to factorize an integer ''n''. We generate, in some way, a large number of integer pairs (''x'', ''y'') for which x \neq \pm y, x^2 \equiv y^2 \pmod, and x^2 \pmod \texty^2 \pmod can be completely factorized over the chosen factor base—that is, all their prime factors are in ''P''.
In practice, several integers ''x'' are found such that x^2 \pmod has all of its prime factors in the pre-chosen factor base. We represent each x^2 \pmod expression as a vector of a matrix with integer entries being the exponents of factors in the factor base. Linear combinations of the rows corresponds to multiplication of these expressions. A linear dependence relation mod 2 among the rows leads to a desired congruence x^2 \equiv y^2 \pmod. This essentially reformulates the problem into a system of linear equations, which can be solved using numerous methods such as Gaussian elimination; in practice advanced methods like the block Lanczos algorithm are used, that take advantage of certain properties of the system.
This congruence may generate the trivial \textstyle n = 1 \cdot n; in this case we try to find another suitable congruence. If repeated attempts to factor fail we can try again using a different factor base.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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