Abstract
This work concerns the general issue of combined optimality in terms of time and space complexity. In this context, we study the problem of (exact) counting resourcelimited and passively mobile nodes in the model of population protocols, in which the space complexity is crucial. The counted nodes are memorylimited anonymous devices (called agents) communicating asynchronously in pairs (according to a fairness condition). Moreover, we assume that these agents are prone to failures so that they cannot be correctly initialized.
This study considers two classical fairness conditions, and for each we investigate the issue of time optimality of counting given the optimal space per agent. In the case of randomly interacting agents (probabilistic fairness), as usual, the convergence time is measured in terms of parallel time (or parallel interactions), which is defined as the number of pairwise interactions until convergence, divided by n (the number of agents). In case of weak fairness, where it is only required that every pair of agents interacts infinitely often, the convergence time is defined in terms of nonnull transitions, i.e, the transitions that affect the states of the interacting agents.
First, assuming probabilistic fairness, we present a "nonguessing" time optimal protocol of O(n log n) expected time given an optimal space of only one bit, and we prove the time optimality of this protocol. Then, for weak fairness, we show that a space optimal (semiuniform) solution cannot converge faster than in bigomega (2^n) time (nonnull transitions). This result, together with the time complexity analysis of an already known space optimal protocol, shows that it is also optimal in time (given the optimal space constrains).
BibTeX  Entry
@InProceedings{aspnes_et_al:LIPIcs:2017:7082,
author = {James Aspnes and Joffroy Beauquier and Janna Burman and Devan Sohier},
title = {{Time and Space Optimal Counting in Population Protocols}},
booktitle = {20th International Conference on Principles of Distributed Systems (OPODIS 2016)},
pages = {13:113:17},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {9783959770316},
ISSN = {18688969},
year = {2017},
volume = {70},
editor = {Panagiota Fatourou and Ernesto Jim{\'e}nez and Fernando Pedone},
publisher = {Schloss DagstuhlLeibnizZentrum fuer Informatik},
address = {Dagstuhl, Germany},
URL = {http://drops.dagstuhl.de/opus/volltexte/2017/7082},
URN = {urn:nbn:de:0030drops70828},
doi = {10.4230/LIPIcs.OPODIS.2016.13},
annote = {Keywords: networks of passively mobile agents/sensors, population protocols, counting, anonymous noninitialized agents, time and space complexity, lower bounds}
}
Keywords: 

networks of passively mobile agents/sensors, population protocols, counting, anonymous noninitialized agents, time and space complexity, lower bounds 
Collection: 

20th International Conference on Principles of Distributed Systems (OPODIS 2016) 
Issue Date: 

2017 
Date of publication: 

06.04.2017 