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Simultaneously flippable edges in triangulations. (English) Zbl 1374.05073
Márquez, Alberto (ed.) et al., Computational geometry. XIV Spanish meeting on computational geometry, EGC 2011, dedicated to Ferran Hurtado on the occasion of his 60th birthday, Alcalá de Henares, Spain, June 27–30, 2011. Revised selected papers. Berlin: Springer (ISBN 978-3-642-34190-8/pbk). Lecture Notes in Computer Science 7579, 138-145 (2012).
Summary: Given a straight-line triangulation $$T$$, an edge $$e$$ in $$T$$ is flippable if $$e$$ is adjacent to two triangles that form a convex quadrilateral. A set of edges $$E$$ in $$T$$ is simultaneously flippable if each edge is flippable and no two edges are adjacent to a common triangle. Intuitively, an edge is flippable if it may be replaced with the other diagonal of its quadrilateral without creating edge-edge intersections, and a set of edges is simultaneously flippable if they may be all be flipped without interferring with each other. We show that every straight-line triangulation on $$n$$ vertices contains at least $$(n-4)/5$$ simultaneously flippable edges. This bound is the best possible, and resolves an open problem by J. Galtier et al. [Int. J. Comput. Geom. Appl. 13, No. 2, 113–133 (2003; Zbl 1058.52005)].
For the entire collection see [Zbl 1253.68016].

##### MSC:
 05C10 Planar graphs; geometric and topological aspects of graph theory
##### Keywords:
planar graph; graph transformation; geometry; combinatorics
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##### References:
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