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While the relationship of time and space is an established topic in traditional centralised com- plexity theory, this is not the case in distributed computing. We aim to remedy this by studying the time and space complexity of algorithms in a weak message-passing model of distributed com- puting. While a constant number of communication rounds implies a constant number of states visited during the execution, the other direction is not clear at all. We show that indeed, there exist non-trivial graph problems that are solvable by constant-space algorithms but that require a non-constant running time. Somewhat surprisingly, this holds even when restricted to the class of only cycle and path graphs. Our work provides us with a new complexity class for distributed computing and raises interesting questions about the existence of further combinations of time and space complexity.
@InProceedings{lempiainen_et_al:LIPIcs.OPODIS.2017.30,
author = {Lempi\"{a}inen, Tuomo and Suomela, Jukka},
title = {{Constant Space and Non-Constant Time in Distributed Computing}},
booktitle = {21st International Conference on Principles of Distributed Systems (OPODIS 2017)},
pages = {30:1--30:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-061-3},
ISSN = {1868-8969},
year = {2018},
volume = {95},
editor = {Aspnes, James and Bessani, Alysson and Felber, Pascal and Leit\~{a}o, Jo\~{a}o},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2017.30},
URN = {urn:nbn:de:0030-drops-86368},
doi = {10.4230/LIPIcs.OPODIS.2017.30},
annote = {Keywords: distributed computing, space complexity, constant-space algorithms, weak models, Thue-Morse sequence}
}