We study C_k-freeness in sparse graphs from a property testing perspective, specifically for graph classes with bounded r-admissibility. Our work is motivated by the large gap between upper and lower bounds in this area: C_k-freeness is known to be testable in planar graphs [Czumaj and Sohler, 2019], but not in graphs with bounded arboricity for k > 3 [Talya Eden et al., 2024]. There are a large number of interesting graph classes that include planar graphs and have bounded arboricity (e.g. classes excluding a minor), calling for a more fine-grained approach to the question of testing C_k-freeness in sparse graph classes. One such approach, inspired by the work of Nesetril and Ossona de Mendez [Nešetřil and {Ossona de Mendez}, 2012], is to consider the graph measure of r-admissibility, which naturally forms a hierarchy of graph families A₁ ⊃ A₂ ⊃ … ⊃ A_∞ where A_r contains all graph classes whose r-admissibility is bounded by some constant. The family A₁ contains classes with bounded arboricity, the class A_∞ contains classes like planar graphs, graphs of bounded degree, and minor-free graphs. Awofeso ηl [Awofeso et al., 2025] recently made progress in this direction. They showed that C₄- and C₅-freeness is testable in A₂. They further showed that C_k-freeness is not testable in A_{⌊k/2⌋ -1} and conjectured that C_k-freeness is testable in A_{⌊k/2⌋}. In this work, we prove this conjecture: C_k-freeness is indeed testable in graphs of bounded ⌊k/2⌋-admissibility.
@InProceedings{awofeso_et_al:LIPIcs.ICALP.2025.15, author = {Awofeso, Christine and Greaves, Patrick and Lachish, Oded and Levi, Amit and Reidl, Felix}, title = {{Testing C\underlinek-Freeness in Bounded Admissibility Graphs}}, booktitle = {52nd International Colloquium on Automata, Languages, and Programming (ICALP 2025)}, pages = {15:1--15:20}, 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.2025.15}, URN = {urn:nbn:de:0030-drops-233926}, doi = {10.4230/LIPIcs.ICALP.2025.15}, annote = {Keywords: Property Testing, Sparse Graphs, Cycle, Admissibility} }
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