How difficult is it to compute the communication complexity of a two-argument total Boolean function f:[N]×[N] → {0,1}, when it is given as an N×N binary matrix? In 2009, Kushilevitz and Weinreb showed that this problem is cryptographically hard, but it is still open whether it is NP-hard. In this work, we show that it is NP-hard to approximate the size (number of leaves) of the smallest constant-round protocol for a two-argument total Boolean function f:[N]×[N] → {0,1}, when it is given as an N×N binary matrix. Along the way to proving this, we show a new deterministic variant of the round elimination lemma, which may be of independent interest.
@InProceedings{hirahara_et_al:LIPIcs.CCC.2021.31, author = {Hirahara, Shuichi and Ilango, Rahul and Loff, Bruno}, title = {{Hardness of Constant-Round Communication Complexity}}, booktitle = {36th Computational Complexity Conference (CCC 2021)}, pages = {31:1--31:30}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-193-1}, ISSN = {1868-8969}, year = {2021}, volume = {200}, editor = {Kabanets, Valentine}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CCC.2021.31}, URN = {urn:nbn:de:0030-drops-143055}, doi = {10.4230/LIPIcs.CCC.2021.31}, annote = {Keywords: NP-completeness, Communication Complexity, Round Elimination Lemma, Meta-Complexity} }
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