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Separations: We introduce a monotone variant of Xor-Sat and show it has exponential monotone circuit complexity. Since Xor-Sat is in NC^2, this improves qualitatively on the monotone vs. non-monotone separation of Tardos (1988). We also show that monotone span programs over R can be exponentially more powerful than over finite fields. These results can be interpreted as separating subclasses of TFNP in communication complexity. Characterizations: We show that the communication (resp. query) analogue of PPA (subclass of TFNP) captures span programs over F_2 (resp. Nullstellensatz degree over F_2). Previously, it was known that communication FP captures formulas (Karchmer - Wigderson, 1988) and that communication PLS captures circuits (Razborov, 1995).
@InProceedings{goos_et_al:LIPIcs.ITCS.2019.38,
author = {G\"{o}\"{o}s, Mika and Kamath, Pritish and Robere, Robert and Sokolov, Dmitry},
title = {{Adventures in Monotone Complexity and TFNP}},
booktitle = {10th Innovations in Theoretical Computer Science Conference (ITCS 2019)},
pages = {38:1--38:19},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-095-8},
ISSN = {1868-8969},
year = {2019},
volume = {124},
editor = {Blum, Avrim},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2019.38},
URN = {urn:nbn:de:0030-drops-101316},
doi = {10.4230/LIPIcs.ITCS.2019.38},
annote = {Keywords: TFNP, Monotone Complexity, Communication Complexity, Proof Complexity}
}