# Problem 10: Multi-Round Communication of Gap-Hamming Distance

Suggested by | Ravi Kumar |
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Source | Kanpur 2006 |

Short link | https://sublinear.info/10 |

Consider the communication problem **Gap-Hamdist**: Alice and Bob are given length $n$ binary strings $x$ and $y$ such that either the Hamming distance $\Delta(x,y) \leq n/2$ or $\Delta(x,y)\geq n/2+\sqrt{n}$. The one-way communication complexity of **Gap-Hamdist** is known to be $\Omega(n)$ [IndykW-03,Woodruff-04]. Recently, a simpler proof was discovered using a reduction from **Index** [JayramKS-07]. Is the multi-round communication complexity also $\Omega(n)$? There is a $\Omega(\sqrt{n})$ lower-bound from a reduction from **Set-Disjointness** but we conjecture that the lower-bound is actually $\Omega(n)$.

If the conjecture is true then it would imply stronger multiple-pass lower bounds for estimating $F_0$ [IndykW-03,Woodruff-04,BarYossefJKST-02] and entropy [BhuvanagiriG-06,ChakrabartiCM-07]. Alternatively, if the conjecture is not true then it would be interesting to see if better multi-pass algorithms exist for $F_0$ and entropy.

## Update

This conjecture was proved in [ChakrabartiR-11], showing that communication complexity of **Gap-Hamdist** is $\Omega(n)$.