Mittagsseminar (in cooperation with A. Steger, D. Steurer and B. Sudakov)

Date and Time: Tuesday, October 01, 2019, 12:15 pm

Duration: 30 minutes

Location: OAT S15/S16/S17

Speaker: Emo Welzl

Bistellar and edge flip graphs of triangulations in the plane: Geometry and connectivity.

The set of all triangulations of a finite point set in the plane attains structure via flips: The graph, where two triangulations are adjacent if one can be obtained from the other by an elementary flip. This is an edge flip for full triangulations, or a bistellar flip for partial triangulations (where non-extreme points can be skipped). It is well-known (Lawson, 1972) that both, the edge flip graph and the bistellar flip graph are connected. For n the number of points and general position assumed, we show that the edge flip graph is (n/2-2)-connected and the bistellar flip graph is (n-3)-connected. Both bounds are tight. This matches the situation for regular triangulations (a subset of the partial triangulations), where (n-3)-connectivity was known through the secondary polytope (Gelfand, Kapranov, Zelevinsky, 1990) and Balinski’s Theorem. We show that the edge flip graph can be covered by 1-skeletons of polytopes of dimension at least n/2-2 (products of associahedra). Similarly, the bistellar flip graph can be covered by 1-skeletons of polytopes of dimension at least n-3 (products of secondary polytopes). (covers joint research with Uli Wagner, IST Austria)

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