Drawings of Complete Multipartite Graphs up to Triangle Flips

Authors Oswin Aichholzer , Man-Kwun Chiu , Hung P. Hoang , Michael Hoffmann , Jan Kynčl , Yannic Maus , Birgit Vogtenhuber , Alexandra Weinberger

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Oswin Aichholzer
  • Technische Universität Graz, Austria
Man-Kwun Chiu
  • Wenzhou-Kean University, Wenzhou, China
Hung P. Hoang
  • Department of Computer Science, ETH Zürich, Switzerland
Michael Hoffmann
  • Department of Computer Science, ETH Zürich, Switzerland
Jan Kynčl
  • Charles University, Prague, Czech Republic
Yannic Maus
  • Technische Universität Graz, Austria
Birgit Vogtenhuber
  • Technische Universität Graz, Austria
Alexandra Weinberger
  • Technische Universität Graz, Austria

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Oswin Aichholzer, Man-Kwun Chiu, Hung P. Hoang, Michael Hoffmann, Jan Kynčl, Yannic Maus, Birgit Vogtenhuber, and Alexandra Weinberger. Drawings of Complete Multipartite Graphs up to Triangle Flips. In 39th International Symposium on Computational Geometry (SoCG 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 258, pp. 6:1-6:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


For a drawing of a labeled graph, the rotation of a vertex or crossing is the cyclic order of its incident edges, represented by the labels of their other endpoints. The extended rotation system (ERS) of the drawing is the collection of the rotations of all vertices and crossings. A drawing is simple if each pair of edges has at most one common point. Gioan’s Theorem states that for any two simple drawings of the complete graph K_n with the same crossing edge pairs, one drawing can be transformed into the other by a sequence of triangle flips (a.k.a. Reidemeister moves of Type 3). This operation refers to the act of moving one edge of a triangular cell formed by three pairwise crossing edges over the opposite crossing of the cell, via a local transformation. We investigate to what extent Gioan-type theorems can be obtained for wider classes of graphs. A necessary (but in general not sufficient) condition for two drawings of a graph to be transformable into each other by a sequence of triangle flips is that they have the same ERS. As our main result, we show that for the large class of complete multipartite graphs, this necessary condition is in fact also sufficient. We present two different proofs of this result, one of which is shorter, while the other one yields a polynomial time algorithm for which the number of needed triangle flips for graphs on n vertices is bounded by O(n^{16}). The latter proof uses a Carathéodory-type theorem for simple drawings of complete multipartite graphs, which we believe to be of independent interest. Moreover, we show that our Gioan-type theorem for complete multipartite graphs is essentially tight in the following sense: For the complete bipartite graph K_{m,n} minus two edges and K_{m,n} plus one edge for any m,n ≥ 4, as well as K_n minus a 4-cycle for any n ≥ 5, there exist two simple drawings with the same ERS that cannot be transformed into each other using triangle flips. So having the same ERS does not remain sufficient when removing or adding very few edges.

Subject Classification

ACM Subject Classification
  • Mathematics of computing → Combinatorics
  • Mathematics of computing → Graph theory
  • Human-centered computing → Graph drawings
  • Simple drawings
  • simple topological graphs
  • complete graphs
  • multipartite graphs
  • k-partite graphs
  • bipartite graphs
  • Gioan’s Theorem
  • triangle flips
  • Reidemeister moves


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