Towards lower bounds on locally testable codes via density arguments

Abstract

The main open problem in the area of locally testable codes (LTCs) is whether there exists an asymptotically good family of LTCs, and to resolve this question, it suffices to consider the case of query complexity 3. We argue that to refute the existence of such an asymptotically good family, it is sufficient to prove that the number of dual codewords of weight at most 3 is super-linear in the blocklength of the code and they are distributed "naturally". The main technical contribution of this paper is an improvement of the combinatorial lemma of Goldreich et al. (Comput Complex 15(3):263-296, 2006) which bounds the rate of 2-query locally decodable codes (LDCs) and is used in state-of-the-art rate bounds for linear LDCs. The lemma of Goldreich et al. bounds the rate of 2-query LDCs of blocklength n in terms of the corruption parameter δ(n)-this is the maximal fraction of corrupted codeword bits for which a (2-query) decoder can recover correctly every message bit (with high probability). Our combinatorial lemma gives non-trivial rate bounds for any corruption parameter δ(n) such that δ(n) · n = ω(1), whereas the previous lemma works only for corruption parameter δ(n) such that δ(n) · n ≥ log n. The study of LDCs with sublinear corruption parameter is also motivated by Dvir's (IEEE conference on computational complexity. IEEE Computer Society, pp 291-298, 2010) observation that sufficiently strong bounds on the rate of such LDCs imply explicit constructions of rigid matrices.