Braess’s paradox is a counterintuitive and undesirable phenomenon, in which for a given graph with prescribed source and sink vertices and cost functions for all edges, removal of edges decreases the cost of a Nash flow from source to sink. The problem of deciding if the phenomenon occurs is generally NP-hard. In this paper, we consider the problem of deciding if, for a given graph with prescribed source and sink vertices, Braess’s paradox does not occur for any cost functions. It is known that this problem can be solved in O(nm²) time for directed graphs, where n and m are the numbers of vertices and edges of the input graph, respectively. In this paper, we propose a faster O(m²) time algorithm solving this problem for directed graphs. Our approach is based on a simple implementation of a known characterization that the subgraph of a given graph induced by all source-sink paths is series-parallel. The faster running time is achieved by speeding up the simple implementation using another characterization that a certain structure is embedded in the given graph. Combined with a known technique, the proposed algorithm can also be used to design a faster O(km²) time algorithm for directed graphs with k source-sink pairs, which improves the previous O(knm²) time algorithm.
@InProceedings{matsubayashi_et_al:OASIcs.ATMOS.2023.12, author = {Matsubayashi, Akira and Saito, Yushi}, title = {{A Faster Algorithm for Recognizing Directed Graphs Invulnerable to Braess’s Paradox}}, booktitle = {23rd Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2023)}, pages = {12:1--12:19}, series = {Open Access Series in Informatics (OASIcs)}, ISBN = {978-3-95977-302-7}, ISSN = {2190-6807}, year = {2023}, volume = {115}, editor = {Frigioni, Daniele and Schiewe, Philine}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2023.12}, URN = {urn:nbn:de:0030-drops-187738}, doi = {10.4230/OASIcs.ATMOS.2023.12}, annote = {Keywords: Braess’s paradox, series-parallel graph, route-induced graph, Nash flow} }
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