We consider the following two algorithmic problems: given a graph G and a subgraph H ⊆ G, decide whether H is an isometric or a geodesically convex subgraph of G. It is relatively easy to see that the problems can be solved by computing the distances between all pairs of vertices. We provide a conditional lower bound showing that, for sparse graphs with n vertices and Θ(n) edges, we cannot expect to solve the problem in O(n^{2-ε}) time for any constant ε > 0. We also show that the problem can be solved in subquadratic time for planar graphs and in near-linear time for graphs of bounded treewidth. Finally, we provide a near-linear time algorithm for the setting where G is a plane graph and H is defined by a few cycles in G.
@InProceedings{cabello:LIPIcs.WADS.2025.12, author = {Cabello, Sergio}, title = {{Testing Whether a Subgraph Is Convex or Isometric}}, booktitle = {19th International Symposium on Algorithms and Data Structures (WADS 2025)}, pages = {12:1--12:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-398-0}, ISSN = {1868-8969}, year = {2025}, volume = {349}, editor = {Morin, Pat and Oh, Eunjin}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.WADS.2025.12}, URN = {urn:nbn:de:0030-drops-242439}, doi = {10.4230/LIPIcs.WADS.2025.12}, annote = {Keywords: convex subgraph, isometric subgraph, plane graph} }
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