New Hardness Results for Diophantine Approximation

  • Authors:
  • Friedrich Eisenbrand;Thomas Rothvoß

  • Affiliations:
  • Institute of Mathematics, EPFL, Lausanne, Switzerland;Institute of Mathematics, EPFL, Lausanne, Switzerland

  • Venue:
  • APPROX '09 / RANDOM '09 Proceedings of the 12th International Workshop and 13th International Workshop on Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques
  • Year:
  • 2009

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Abstract

We revisit simultaneous Diophantine approximation , a classical problem from the geometry of numbers which has many applications in algorithms and complexity. The input to the decision version of this problem consists of a rational vector *** *** *** n , an error bound *** and a denominator bound N *** *** + . One has to decide whether there exists an integer, called the denominator Q with 1 ≤ Q ≤ N such that the distance of each number Q ·*** i to its nearest integer is bounded by *** . Lagarias has shown that this problem is NP-complete and optimization versions have been shown to be hard to approximate within a factor n c / loglogn for some constant c 0. We strengthen the existing hardness results and show that the optimization problem of finding the smallest denominator Q *** *** + such that the distances of Q ·*** i to the nearest integer are bounded by *** is hard to approximate within a factor 2 n unless ${\textrm{P}} = {\rm NP}$. We then outline two further applications of this strengthening: We show that a directed version of Diophantine approximation is also hard to approximate. Furthermore we prove that the mixing set problem with arbitrary capacities is NP-hard. This solves an open problem raised by Conforti, Di Summa and Wolsey.