
Revised: March 24, 2017
Published: December 28, 2018
Abstract: [Plain Text Version]
The unbounded-error communication complexity of a Boolean function F is the limit of the ϵ-error randomized complexity of F as ϵ→1/2. Communication complexity with weakly unbounded error is defined similarly but with an additive penalty term that depends on 1/2−ϵ. Explicit functions are known whose two-party communication complexity with unbounded error is logarithmic compared to their complexity with weakly unbounded error. Chattopadhyay and Mande (ECCC TR16-095, Theory of Computing 2018) recently generalized this exponential separation to the number-on-the-forehead multiparty model. We show how to derive such an exponential separation from known two-party work, achieving a quantitative improvement along the way. We present several proofs here, some as short as half a page.
In more detail, we construct a k-party communication problem F:({0,1}n)k→{0,1} that has complexity O(logn) with unbounded error and Ω(√n/4k) with weakly unbounded error, reproducing the bounds of Chattopadhyay and Mande. In addition, we prove a quadratically stronger separation of O(logn) versus Ω(n/4k) using a nonconstructive argument.
A preliminary version of this paper appeared in ECCC, Report TR16-138, 2016.