Exact polynomial factorization by approximate high degree algebraic numbers

For factoring polynomials in two variables with rational coefficients, an algorithm using transcendental evaluation was presented by Hulst and Lenstra. In their algorithm, transcendence measure was computed. However, a constant c is necessary to compute the transcendence measure. The size of c involved the transcendence measure can influence the efficiency of the algorithm greatly. In this paper, we overcome the problem arising in Hulst and Lenstra's algorithm and propose a new polynomial time algorithm for factoring bivariate polynomials with rational coefficients. Using an approximate algebraic number of high degree instead of a variable of a bivariate polynomial, we can get a univariate one. A factor of the resulting univariate polynomial can then be obtained by a numerical root finder and the purely numerical LLL algorithm. The high degree of the algebraic number guarantees that this factor corresponds to a factor of the original bivariate polynomial. We prove that our algorithm saves a (log2(mn))2+ε factor in bit-complexity comparing with the algorithm presented by Hulst and Lenstra, where (n, m) represents the bi-degree of the polynomial to be factored. We also demonstrate on many significant experiments that our algorithm is practical. Moreover our algorithm can be generalized to polynomials with variables more than two.