An Optimal Lower Bound on the Communication Complexity of GapHammingDistance
Abstract
We prove an optimal $\Omega(n)$ lower bound on the randomized communication complexity of the muchstudied GapHammingDistance problem. As a consequence, we obtain essentially optimal multipass space lower bounds in the data stream model for a number of fundamental problems, including the estimation of frequency moments. The GapHammingDistance problem is a communication problem, wherein Alice and Bob receive $n$bit strings $x$ and $y$, respectively. They are promised that the Hamming distance between $x$ and $y$ is either at least $n/2+\sqrt{n}$ or at most $n/2\sqrt{n}$, and their goal is to decide which of these is the case. Since the formal presentation of the problem by Indyk and Woodruff (FOCS, 2003), it had been conjectured that the naive protocol, which uses $n$ bits of communication, is asymptotically optimal. The conjecture was shown to be true in several special cases, e.g., when the communication is deterministic, or when the number of rounds of communication is limited. The proof of our aforementioned result, which settles this conjecture fully, is based on a new geometric statement regarding correlations in Gaussian space, related to a result of C. Borell (1985). To prove this geometric statement, we show that random projections of nottoosmall sets in Gaussian space are close to a mixture of translated normal variables.
 Publication:

arXiv eprints
 Pub Date:
 September 2010
 arXiv:
 arXiv:1009.3460
 Bibcode:
 2010arXiv1009.3460C
 Keywords:

 Computer Science  Computational Complexity;
 Mathematics  Metric Geometry