Certification of the QR factor R and of lattice basis reducedness

  • Authors:
  • Gilles Villard

  • Affiliations:
  • CNRS, Laboratoire LIP (CNRS, ENSL, INRIA, UCBL), École Normale Supérieure de Lyon, France

  • Venue:
  • Proceedings of the 2007 international symposium on Symbolic and algebraic computation
  • Year:
  • 2007

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Abstract

Given a lattice basis of n vectors in Zn, we propose an algorithm using 12n3+O(n2) floating point operations for checking whether the basis is LLL-reduced. If the basis is reduced then the algorithm will hopefully answer "yes". If the basis is not reduced, or if the precision used is not sufficient with respect to n, and to the numerical properties of the basis, the algorithm will answer "failed". Hence a positive answer is a rigorous certificate. For implementing the certificate itself, we propose a oating point algorithm for computing (certified) error bounds for the R factor of the QR factorization. This algorithm takes into account all possible approximation and rounding errors. The certificate may be implemented using matrix library routines only. We report experiments that show that for a reduced basis of adequate dimension and quality the certificate succeeds, and establish the effectiveness of the certificate. This effectiveness is applied for certifying the output of fastest existing floating point heuristics of LLL reduction, without slowing down the whole process.