Fix is_cactus() for small disconnected graphs #40762
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dcoudert
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Sep 7, 2025
sagemathgh-40762: Fix is_cactus() for small disconnected graphs This fixes [`is_cactus()`](https://doc-develop--sagemath.netlify.app/htm l/en/reference/graphs/sage/graphs/graph#sage.graphs.graph.Graph.is_cactu s) erroneously returning `True` for the following small disconnected graphs: [`Graph(2)`](https://houseofgraphs.org/graphs/19653), [`Graph(3)`](https://houseofgraphs.org/graphs/51081), [`Graph('BG')`](https://houseofgraphs.org/graphs/49432). I also added a special case for trees as a small performance improvement. ### 📝 Checklist - [x] The title is concise and informative. - [x] The description explains in detail what this PR is about. - [x] I have created tests covering the changes. - [x] I have updated the documentation and checked the documentation preview. URL: sagemath#40762 Reported by: Lennard Hofmann Reviewer(s): David Coudert
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This fixes
is_cactus()erroneously returningTruefor the following small disconnected graphs:Graph(2),Graph(3),Graph('BG').I also added a special case for trees as a small performance improvement.
📝 Checklist