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Direct Sum Fails for Zero-Error Average Communication

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Abstract

We show that in the model of zero-error communication complexity, direct sum fails for average communication complexity as well as for external information complexity. Our example also refutes a version of a conjecture by Braverman et al. that in the zero-error case amortized communication complexity equals external information complexity. In our examples the underlying distributions do not have full support. One interpretation of a distribution of non full support is as a promise given to the players (the players have a guarantee on their inputs). This brings up the issue of promise versus non-promise problems in this context.

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Notes

  1. This is because in any non trivial case the communication complexity is at least one.

  2. The limit exists since the corresponding sequence is Cauchy.

  3. Even for fixed (xy) the transcript T(xy) is a random variable that depends on the private coins.

  4. \(\mathrm{supp}(\mu )\) is the set of all \((x,y)\in \mathcal{X}\times \mathcal{Y}\) such that \(\mu (x,y)>0\).

  5. By \(\pi (x,y,r) = f(x,y)\) we mean that knowledge of the transcript and the public randomness yields knowledge of f as in Sect. 2.1.

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Correspondence to Shay Moran.

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Gillat Kol: Research at the IAS supported by The Fund For Math and the Weizmann Institute of Science National Postdoctoral Award Program for Advancing Women in Science. Part of this work was done while the author was visiting the Simons Institute for the Theory of Computing, Berkeley, CA, and the Technion, Israel.

Shay Moran and Amir Shpilka: The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement number 257575.

Amir Yehudayoff: Horev fellow—supported by the Taub foundation. Research is also supported by ISF and BSF.

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Kol, G., Moran, S., Shpilka, A. et al. Direct Sum Fails for Zero-Error Average Communication. Algorithmica 76, 782–795 (2016). https://doi.org/10.1007/s00453-016-0144-9

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