Vector rational number reconstruction

  • Authors:
  • Curtis Bright;Arne Storjohann

  • Affiliations:
  • University of Waterloo, Waterloo, ON, Canada;University of Waterloo, Waterloo, ON, Canada

  • Venue:
  • Proceedings of the 36th international symposium on Symbolic and algebraic computation
  • Year:
  • 2011

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Abstract

The final step of some algebraic algorithms is to reconstruct the common denominator d of a collection of rational numbers (ni/d)1≤ i≤ n from their images (ai)1≤ i≤ n mod M, subject to a condition such as 0 d ≤ N and Ni}≤ N for a given magnitude bound N. Applying elementwise rational number reconstruction requires that M ∈ Ω(N2). Using the gradual sublattice reduction algorithm of van Hoeij and Novocin, we show how to perform the reconstruction efficiently even when the modulus satisfies a considerably smaller magnitude bound M ∈ Ω(N1+1/c) for c a small constant, for example 2 ≤ c ≤ 5. Assuming c ∈ O(1) the cost of the approach is O(n(log M)3) bit operations using the original LLL lattice reduction algorithm, but is reduced to O(n(log M)2) bit operations by incorporating the L2 variant of Nguyen and Stehle. As an application, we give a robust method for reconstructing the rational solution vector of a linear system from its image, such as obtained by a solver using p-adic lifting.