New separations results for external information

Abstract

We obtain new separation results for the two-party external information complexity of Boolean functions. The external information complexity of a function f(x,y) is the minimum amount of information a two-party protocol computing f must reveal to an outside observer about the input. We prove an exponential separation between external and internal information complexity, which is the best possible; previously no separation was known. We use this result in order to then prove a near-quadratic separation between amortized zero-error communication complexity and external information complexity for total functions, disproving a conjecture of the first author. Finally, we prove a matching upper bound showing that our separation result is tight.