## Abstract

The Nearest Codeword Problem (NCP) is a basic algorithmic question in the theory of error-correcting codes. Given a point v F^{n}_{2} and a linear space L F^{n}_{2} of dimension k NCP asks to find a point l L that minimizes the (Hamming) distance from v. It is well-known that the nearest codeword problem is NP-hard. Therefore approximation algorithms are of interest. The best efficient approximation algorithms for the NCP to date are due to Berman and Karpinski. They are a deterministic algorithm that achieves an approximation ratio of O(k/c) for an arbitrary constant c, and a randomized algorithm that achieves an approximation ratio of O(k/ log n). In this paper we present new deterministic algorithms for approximating the NCP that improve substantially upon the earlier work. Specifically, we obtain: – A polynomial time O(n/ log n)-approximation algorithm; – An n^{O}(s^{)} time O(k log^{(}s^{)} n/ log n)-approximation algorithm, where log^{(}s^{)} n stands for s iterations of log, e.g., log^{(2)} n = log log n; – An n^{O}^{(log* n)} time O(k/ log n)-approximation algorithm. We also initiate a study of the following Remote Point Problem (RPP). Given a linear space L F^{n}_{2} of dimension k RPP asks to find a point v F^{n}_{2} that is far from L. We say that an algorithm achieves a remoteness of r for the RPP if it always outputs a point v that is at least r-far from L. In this paper we present a deterministic polynomial time algorithm that achieves a remoteness of Ω(n log k/k) for all k ≤ n/2. We motivate the remote point problem by relating it to both the nearest codeword problem and the matrix rigidity approach to circuit lower bounds in computational complexity theory.

Original language | English (US) |
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Journal | Dagstuhl Seminar Proceedings |

Volume | 9421 |

State | Published - 2010 |

Event | Algebraic Methods in Computational Complexity 2009 - Wadern, Germany Duration: Oct 11 2009 → Oct 16 2009 |

## All Science Journal Classification (ASJC) codes

- Software
- Hardware and Architecture
- Control and Systems Engineering