# Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips)

Wagner U, Welzl E. 2020. Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips). 36th International Symposium on Computational Geometry. SoCG: Symposium on Computational Geometry, LIPIcs, vol. 164, 67:1-67:16.

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Author
Wagner, UliISTA ; Welzl, Emo
Department
Series Title
LIPIcs
Abstract
Given a finite point set P in general position in the plane, a full triangulation is a maximal straight-line embedded plane graph on P. A partial triangulation on P is a full triangulation of some subset P' of P containing all extreme points in P. A bistellar flip on a partial triangulation either flips an edge, removes a non-extreme point of degree 3, or adds a point in P ⧵ P' as vertex of degree 3. The bistellar flip graph has all partial triangulations as vertices, and a pair of partial triangulations is adjacent if they can be obtained from one another by a bistellar flip. The goal of this paper is to investigate the structure of this graph, with emphasis on its connectivity. For sets P of n points in general position, we show that the bistellar flip graph is (n-3)-connected, thereby answering, for sets in general position, an open questions raised in a book (by De Loera, Rambau, and Santos) and a survey (by Lee and Santos) on triangulations. This matches the situation for the subfamily of regular triangulations (i.e., partial triangulations obtained by lifting the points and projecting the lower convex hull), where (n-3)-connectivity has been known since the late 1980s through the secondary polytope (Gelfand, Kapranov, Zelevinsky) and Balinski’s Theorem. Our methods also yield the following results (see the full version [Wagner and Welzl, 2020]): (i) The bistellar flip graph can be covered by graphs of polytopes of dimension n-3 (products of secondary polytopes). (ii) A partial triangulation is regular, if it has distance n-3 in the Hasse diagram of the partial order of partial subdivisions from the trivial subdivision. (iii) All partial triangulations are regular iff the trivial subdivision has height n-3 in the partial order of partial subdivisions. (iv) There are arbitrarily large sets P with non-regular partial triangulations, while every proper subset has only regular triangulations, i.e., there are no small certificates for the existence of non-regular partial triangulations (answering a question by F. Santos in the unexpected direction).
Publishing Year
Date Published
2020-06-01
Proceedings Title
36th International Symposium on Computational Geometry
Volume
164
Article Number
67:1 - 67:16
Conference
SoCG: Symposium on Computational Geometry
Conference Location
Zürich, Switzerland
Conference Date
2020-06-22 – 2020-06-26
ISBN
ISSN
IST-REx-ID

### Cite this

Wagner U, Welzl E. Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips). In: 36th International Symposium on Computational Geometry. Vol 164. Schloss Dagstuhl - Leibniz-Zentrum für Informatik; 2020. doi:10.4230/LIPIcs.SoCG.2020.67
Wagner, U., & Welzl, E. (2020). Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips). In 36th International Symposium on Computational Geometry (Vol. 164). Zürich, Switzerland: Schloss Dagstuhl - Leibniz-Zentrum für Informatik. https://doi.org/10.4230/LIPIcs.SoCG.2020.67
Wagner, Uli, and Emo Welzl. “Connectivity of Triangulation Flip Graphs in the Plane (Part II: Bistellar Flips).” In 36th International Symposium on Computational Geometry, Vol. 164. Schloss Dagstuhl - Leibniz-Zentrum für Informatik, 2020. https://doi.org/10.4230/LIPIcs.SoCG.2020.67.
U. Wagner and E. Welzl, “Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips),” in 36th International Symposium on Computational Geometry, Zürich, Switzerland, 2020, vol. 164.
Wagner U, Welzl E. 2020. Connectivity of triangulation flip graphs in the plane (Part II: Bistellar flips). 36th International Symposium on Computational Geometry. SoCG: Symposium on Computational Geometry, LIPIcs, vol. 164, 67:1-67:16.
Wagner, Uli, and Emo Welzl. “Connectivity of Triangulation Flip Graphs in the Plane (Part II: Bistellar Flips).” 36th International Symposium on Computational Geometry, vol. 164, 67:1-67:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik, 2020, doi:10.4230/LIPIcs.SoCG.2020.67.
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2020-06-23
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arXiv 2003.13557