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A quasi-polynomial time algorithm for multi-arrival on tree-like multigraphs

Authors: Ghorbani, Ebrahim; Hoff, Jonah Leander; Mnich, Matthias;

A quasi-polynomial time algorithm for multi-arrival on tree-like multigraphs

Abstract

Propp machines, or rotor-router models, are a classic tool to simulate random systems in forms of Markov chains by deterministic systems. To this end, the nodes of the Markov chain are replaced by switching nodes, which maintain a queue over their outgoing arcs, and a particle sent through the system traverses the top arc of the queue which is then moved to the end of the queue and the particle arrives at the next node. A key question to answer about such systems is whether a single particle can reach a particular target node, given as input an initial configuration of the queues at all switching nodes. This question was introduced by Dohrau et al. (2017) under the name of Arrival. A major open question is whether Arrival can be solved in polynomial time, as it is known to lie in NP ∩co-NP; yet the fastest known algorithm for general instances takes subexponential time (Gärtner et al., ICALP 2021). We consider a generalized version of Arrival introduced by Auger et al. (RP 2023), which requires routing multiple (potentially exponentially many) particles through a rotor graph. The Multi-Arrival problem is to determine the particle configuration that results from moving all particles from a given initial configuration to sinks. Auger et al. showed that for path-like rotor graphs with a certain uniform rotor order, the problem can be solved in polynomial time. Our main result is a quasi-polynomial-time algorithm for Multi-Arrival on tree-like rotor graphs for arbitrary rotor orders. Tree-like rotor graphs are directed multigraphs which can be obtained from undirected trees by replacing each edge by an arbitrary number of arcs in either or both directions. For trees of bounded contracted height, such as paths, the algorithm runs in polynomial time and thereby generalizes the result by Auger et al.. Moreover, we give a polynomial-time algorithm for Multi-Arrival on tree-like rotor graphs without parallel arcs.

Keywords

Fixed-Parameter Tractability, Computer Science, Information and General Works::004: Computer Sciences, Rotor-routing, Arrival, Path-Like Multigraph, Tree-like Multigraph, Natural Sciences and Mathematics::519: Applied Mathematics, Probabilities, Computer Science, Information and General Works::005: Computer Programming, Programs, Data and Security::005.1: Programming, 004, 510, ddc: ddc:004

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
0
Average
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