Deterministic Reduction of Integer Nonsingular Linear System Solving to Matrix Multiplication

We present a deterministic reduction to matrix multiplication for the problem of linear system solving: given as input a nonsingular A \in \Z^n \times n and b \in \Z^n \times 1 , compute A^-1 b. We give an algorithm that computes the minimal integer e such that all denominators of the entries in 2^eA^-1 are relatively prime to 2. Then, for a b that has entries with bitlength O(n) times as large as the bitlength of entries in A, we give an algorithm to produce the 2-adic expansion of 2^eA^-1 b up to a precision high enough such that A^-1 b over \Q can be recovered using rational number reconstruction. Both e and the 2-adic expansion can be computed in O(\MM(n,łog n + łog ||A||) \times (łog n) (łog n + łoglog ||A||)) bit operations. Here, ||A||= \max_ij |A_ij | and \MM(n,d) is the cost to multiply together, modulo 2^d, two n \times n integer matrices. Our approach is based on the previously known reductions of linear system solving to matrix multiplication which use randomization to find an integer lifting modulus X that is relatively prime to \det A. Here, we derandomize by first computing a permutation P, a unit upper triangular M, and a diagonal S with \det S a power of two, such that U := APMS^-1 is an integer matrix with 2 \perp \det U. This allows our modulus X to be chosen a power of 2.