Flows and colorings are disparate concepts in graph algorithms - the former is tractable while the latter is intractable. Tutte [Tutte, 1954; Tutte, 1966] introduced the concept of nowhere-zero flows to unify these two concepts. Jaeger [Jaeger, 1976] showed that nowhere-zero flows are equivalent to cut-balanced orientations. Motivated by connections between nowhere-zero flows, cut-balanced orientations, Nash-Williams' well-balanced orientations, and postman problems, we study optimization versions of nowhere-zero flows and cut-balanced orientations. Given a bidirected graph with asymmetric costs on two orientations of each edge, we study the min cost nowhere-zero k-flow problem and min cost k-cut-balanced orientation problem. We show that both problems are NP-hard to approximate within any finite factor. Given the strong inapproximability result, we design bicriteria approximations for both problems: we obtain a (6,6)-approximation to the min cost nowhere-zero k-flow and a (k,6)-approximation to the min cost k-cut-balanced orientation. For the case of symmetric costs (where the costs of both orientations are the same for every edge), we show that the nowhere-zero k-flow problem remains NP-hard and admits a 3-approximation.
@InProceedings{chandrasekaran_et_al:LIPIcs.ICALP.2025.46, author = {Chandrasekaran, Karthekeyan and Liu, Siyue and Ravi, R.}, title = {{Minimum Cost Nowhere-Zero Flows and Cut-Balanced Orientations}}, booktitle = {52nd International Colloquium on Automata, Languages, and Programming (ICALP 2025)}, pages = {46:1--46:21}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-372-0}, ISSN = {1868-8969}, year = {2025}, volume = {334}, editor = {Censor-Hillel, Keren and Grandoni, Fabrizio and Ouaknine, Jo\"{e}l and Puppis, Gabriele}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2025.46}, URN = {urn:nbn:de:0030-drops-234238}, doi = {10.4230/LIPIcs.ICALP.2025.46}, annote = {Keywords: Nowhere-zero Flows, Cut-balanced orientations, Bicriteria approximation algorithms} }
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