# 15 Search Results for "Aichholzer, Oswin"

Document
##### 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, and Alexandra Weinberger

Published in: LIPIcs, Volume 258, 39th International Symposium on Computational Geometry (SoCG 2023)

##### Abstract
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.

##### Cite as

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)

```@InProceedings{aichholzer_et_al:LIPIcs.SoCG.2023.6,
author =	{Aichholzer, Oswin and Chiu, Man-Kwun and Hoang, Hung P. and Hoffmann, Michael and Kyn\v{c}l, Jan and Maus, Yannic and Vogtenhuber, Birgit and Weinberger, Alexandra},
title =	{{Drawings of Complete Multipartite Graphs up to Triangle Flips}},
booktitle =	{39th International Symposium on Computational Geometry (SoCG 2023)},
pages =	{6:1--6:16},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-273-0},
ISSN =	{1868-8969},
year =	{2023},
volume =	{258},
editor =	{Chambers, Erin W. and Gudmundsson, Joachim},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2023.6},
URN =		{urn:nbn:de:0030-drops-178563},
doi =		{10.4230/LIPIcs.SoCG.2023.6},
annote =	{Keywords: Simple drawings, simple topological graphs, complete graphs, multipartite graphs, k-partite graphs, bipartite graphs, Gioan’s Theorem, triangle flips, Reidemeister moves}
}```
Document
##### Hardness of Token Swapping on Trees

Authors: Oswin Aichholzer, Erik D. Demaine, Matias Korman, Anna Lubiw, Jayson Lynch, Zuzana Masárová, Mikhail Rudoy, Virginia Vassilevska Williams, and Nicole Wein

Published in: LIPIcs, Volume 244, 30th Annual European Symposium on Algorithms (ESA 2022)

##### Abstract
Given a graph where every vertex has exactly one labeled token, how can we most quickly execute a given permutation on the tokens? In (sequential) token swapping, the goal is to use the shortest possible sequence of swaps, each of which exchanges the tokens at the two endpoints of an edge of the graph. In parallel token swapping, the goal is to use the fewest rounds, each of which consists of one or more swaps on the edges of a matching. We prove that both of these problems remain NP-hard when the graph is restricted to be a tree. These token swapping problems have been studied by disparate groups of researchers in discrete mathematics, theoretical computer science, robot motion planning, game theory, and engineering. Previous work establishes NP-completeness on general graphs (for both problems), constant-factor approximation algorithms, and some poly-time exact algorithms for simple graph classes such as cliques, stars, paths, and cycles. Sequential and parallel token swapping on trees were first studied over thirty years ago (as "sorting with a transposition tree") and over twenty-five years ago (as "routing permutations via matchings"), yet their complexities were previously unknown. We also show limitations on approximation of sequential token swapping on trees: we identify a broad class of algorithms that encompass all three known polynomial-time algorithms that achieve the best known approximation factor (which is 2) and show that no such algorithm can achieve an approximation factor less than 2.

##### Cite as

Oswin Aichholzer, Erik D. Demaine, Matias Korman, Anna Lubiw, Jayson Lynch, Zuzana Masárová, Mikhail Rudoy, Virginia Vassilevska Williams, and Nicole Wein. Hardness of Token Swapping on Trees. In 30th Annual European Symposium on Algorithms (ESA 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 244, pp. 3:1-3:15, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2022)

```@InProceedings{aichholzer_et_al:LIPIcs.ESA.2022.3,
author =	{Aichholzer, Oswin and Demaine, Erik D. and Korman, Matias and Lubiw, Anna and Lynch, Jayson and Mas\'{a}rov\'{a}, Zuzana and Rudoy, Mikhail and Vassilevska Williams, Virginia and Wein, Nicole},
title =	{{Hardness of Token Swapping on Trees}},
booktitle =	{30th Annual European Symposium on Algorithms (ESA 2022)},
pages =	{3:1--3:15},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-247-1},
ISSN =	{1868-8969},
year =	{2022},
volume =	{244},
editor =	{Chechik, Shiri and Navarro, Gonzalo and Rotenberg, Eva and Herman, Grzegorz},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2022.3},
URN =		{urn:nbn:de:0030-drops-169413},
doi =		{10.4230/LIPIcs.ESA.2022.3},
annote =	{Keywords: Sorting, Token swapping, Trees, NP-hard, Approximation}
}```
Document
##### Twisted Ways to Find Plane Structures in Simple Drawings of Complete Graphs

Authors: Oswin Aichholzer, Alfredo García, Javier Tejel, Birgit Vogtenhuber, and Alexandra Weinberger

Published in: LIPIcs, Volume 224, 38th International Symposium on Computational Geometry (SoCG 2022)

##### Abstract
Simple drawings are drawings of graphs in which the edges are Jordan arcs and each pair of edges share at most one point (a proper crossing or a common endpoint). We introduce a special kind of simple drawings that we call generalized twisted drawings. A simple drawing is generalized twisted if there is a point O such that every ray emanating from O crosses every edge of the drawing at most once and there is a ray emanating from O which crosses every edge exactly once. Via this new class of simple drawings, we show that every simple drawing of the complete graph with n vertices contains Ω(n^{1/2}) pairwise disjoint edges and a plane path of length Ω((log n)/(log log n)). Both results improve over previously known best lower bounds. On the way we show several structural results about and properties of generalized twisted drawings. We further present different characterizations of generalized twisted drawings, which might be of independent interest.

##### Cite as

Oswin Aichholzer, Alfredo García, Javier Tejel, Birgit Vogtenhuber, and Alexandra Weinberger. Twisted Ways to Find Plane Structures in Simple Drawings of Complete Graphs. In 38th International Symposium on Computational Geometry (SoCG 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 224, pp. 5:1-5:18, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2022)

```@InProceedings{aichholzer_et_al:LIPIcs.SoCG.2022.5,
author =	{Aichholzer, Oswin and Garc{\'\i}a, Alfredo and Tejel, Javier and Vogtenhuber, Birgit and Weinberger, Alexandra},
title =	{{Twisted Ways to Find Plane Structures in Simple Drawings of Complete Graphs}},
booktitle =	{38th International Symposium on Computational Geometry (SoCG 2022)},
pages =	{5:1--5:18},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-227-3},
ISSN =	{1868-8969},
year =	{2022},
volume =	{224},
editor =	{Goaoc, Xavier and Kerber, Michael},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2022.5},
URN =		{urn:nbn:de:0030-drops-160136},
doi =		{10.4230/LIPIcs.SoCG.2022.5},
annote =	{Keywords: Simple drawings, simple topological graphs, disjoint edges, plane matching, plane path}
}```
Document
##### Edge Partitions of Complete Geometric Graphs

Authors: Oswin Aichholzer, Johannes Obenaus, Joachim Orthaber, Rosna Paul, Patrick Schnider, Raphael Steiner, Tim Taubner, and Birgit Vogtenhuber

Published in: LIPIcs, Volume 224, 38th International Symposium on Computational Geometry (SoCG 2022)

##### Abstract
In this paper, we disprove the long-standing conjecture that any complete geometric graph on 2n vertices can be partitioned into n plane spanning trees. Our construction is based on so-called bumpy wheel sets. We fully characterize which bumpy wheels can and in particular which cannot be partitioned into plane spanning trees (or even into arbitrary plane subgraphs). Furthermore, we show a sufficient condition for generalized wheels to not admit a partition into plane spanning trees, and give a complete characterization when they admit a partition into plane spanning double stars. Finally, we initiate the study of partitions into beyond planar subgraphs, namely into k-planar and k-quasi-planar subgraphs and obtain first bounds on the number of subgraphs required in this setting.

##### Cite as

Oswin Aichholzer, Johannes Obenaus, Joachim Orthaber, Rosna Paul, Patrick Schnider, Raphael Steiner, Tim Taubner, and Birgit Vogtenhuber. Edge Partitions of Complete Geometric Graphs. In 38th International Symposium on Computational Geometry (SoCG 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 224, pp. 6:1-6:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2022)

```@InProceedings{aichholzer_et_al:LIPIcs.SoCG.2022.6,
author =	{Aichholzer, Oswin and Obenaus, Johannes and Orthaber, Joachim and Paul, Rosna and Schnider, Patrick and Steiner, Raphael and Taubner, Tim and Vogtenhuber, Birgit},
title =	{{Edge Partitions of Complete Geometric Graphs}},
booktitle =	{38th International Symposium on Computational Geometry (SoCG 2022)},
pages =	{6:1--6:16},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-227-3},
ISSN =	{1868-8969},
year =	{2022},
volume =	{224},
editor =	{Goaoc, Xavier and Kerber, Michael},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2022.6},
URN =		{urn:nbn:de:0030-drops-160141},
doi =		{10.4230/LIPIcs.SoCG.2022.6},
annote =	{Keywords: edge partition, complete geometric graph, plane spanning tree, wheel set}
}```
Document
##### Drawing Graphs with Circular Arcs and Right-Angle Crossings

Authors: Steven Chaplick, Henry Förster, Myroslav Kryven, and Alexander Wolff

Published in: LIPIcs, Volume 162, 17th Scandinavian Symposium and Workshops on Algorithm Theory (SWAT 2020)

##### Abstract
In a RAC drawing of a graph, vertices are represented by points in the plane, adjacent vertices are connected by line segments, and crossings must form right angles. Graphs that admit such drawings are RAC graphs. RAC graphs are beyond-planar graphs and have been studied extensively. In particular, it is known that a RAC graph with n vertices has at most 4n-10 edges. We introduce a superclass of RAC graphs, which we call arc-RAC graphs. A graph is arc-RAC if it admits a drawing where edges are represented by circular arcs and crossings form right angles. We provide a Turán-type result showing that an arc-RAC graph with n vertices has at most 14n-12 edges and that there are n-vertex arc-RAC graphs with 4.5n - O(√n) edges.

##### Cite as

Steven Chaplick, Henry Förster, Myroslav Kryven, and Alexander Wolff. Drawing Graphs with Circular Arcs and Right-Angle Crossings. In 17th Scandinavian Symposium and Workshops on Algorithm Theory (SWAT 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 162, pp. 21:1-21:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

```@InProceedings{chaplick_et_al:LIPIcs.SWAT.2020.21,
author =	{Chaplick, Steven and F\"{o}rster, Henry and Kryven, Myroslav and Wolff, Alexander},
title =	{{Drawing Graphs with Circular Arcs and Right-Angle Crossings}},
booktitle =	{17th Scandinavian Symposium and Workshops on Algorithm Theory (SWAT 2020)},
pages =	{21:1--21:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-150-4},
ISSN =	{1868-8969},
year =	{2020},
volume =	{162},
editor =	{Albers, Susanne},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SWAT.2020.21},
URN =		{urn:nbn:de:0030-drops-122687},
doi =		{10.4230/LIPIcs.SWAT.2020.21},
annote =	{Keywords: circular arcs, right-angle crossings, edge density, charging argument}
}```
Document
##### On Implementing Straight Skeletons: Challenges and Experiences

Authors: Günther Eder, Martin Held, and Peter Palfrader

Published in: LIPIcs, Volume 164, 36th International Symposium on Computational Geometry (SoCG 2020)

##### Abstract
We present Cgal implementations of two algorithms for computing straight skeletons in the plane, based on exact arithmetic. One code, named Surfer2, can handle multiplicatively weighted planar straight-line graphs (PSLGs) while our second code, Monos, is specifically targeted at monotone polygons. Both codes are available on GitHub. We discuss algorithmic as well as implementational and engineering details of both codes. Furthermore, we present the results of an extensive performance evaluation in which we compared Surfer2 and Monos to the straight-skeleton package included in Cgal. It is not surprising that our special-purpose code Monos outperforms Cgal’s straight-skeleton implementation. But our tests provide ample evidence that also Surfer2 can be expected to be faster and to consume significantly less memory than the Cgal code. And, of course, Surfer2 is more versatile because it can handle multiplicative weights and general PSLGs as input. Thus, Surfer2 currently is the fastest and most general straight-skeleton code available.

##### Cite as

Günther Eder, Martin Held, and Peter Palfrader. On Implementing Straight Skeletons: Challenges and Experiences. In 36th International Symposium on Computational Geometry (SoCG 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 164, pp. 38:1-38:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

```@InProceedings{eder_et_al:LIPIcs.SoCG.2020.38,
author =	{Eder, G\"{u}nther and Held, Martin and Palfrader, Peter},
title =	{{On Implementing Straight Skeletons: Challenges and Experiences}},
booktitle =	{36th International Symposium on Computational Geometry (SoCG 2020)},
pages =	{38:1--38:17},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-143-6},
ISSN =	{1868-8969},
year =	{2020},
volume =	{164},
editor =	{Cabello, Sergio and Chen, Danny Z.},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2020.38},
URN =		{urn:nbn:de:0030-drops-121964},
doi =		{10.4230/LIPIcs.SoCG.2020.38},
annote =	{Keywords: weighted straight skeleton, implementation, algorithm engineering, experiments, Cgal, Core}
}```
Document
Media Exposition
##### Step-By-Step Straight Skeletons (Media Exposition)

Authors: Günther Eder, Martin Held, and Peter Palfrader

Published in: LIPIcs, Volume 164, 36th International Symposium on Computational Geometry (SoCG 2020)

##### Abstract
We present two software packages for computing straight skeletons: Monos, our implementation of an algorithm by Biedl et al. (2015), computes the straight skeleton of a monotone input polygon, and Surfer2 implements a generalization of an algorithm by Aichholzer and Aurenhammer (1998) to handle multiplicatively-weighted planar straight-line graphs as input. The graphical user interfaces that ship with our codes support step-by-step computations, where each event can be investigated and studied by the user. This makes them a canonical candidate for educational purposes and detailed event analyses. Both codes are freely available on GitHub.

##### Cite as

Günther Eder, Martin Held, and Peter Palfrader. Step-By-Step Straight Skeletons (Media Exposition). In 36th International Symposium on Computational Geometry (SoCG 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 164, pp. 76:1-76:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

```@InProceedings{eder_et_al:LIPIcs.SoCG.2020.76,
author =	{Eder, G\"{u}nther and Held, Martin and Palfrader, Peter},
title =	{{Step-By-Step Straight Skeletons}},
booktitle =	{36th International Symposium on Computational Geometry (SoCG 2020)},
pages =	{76:1--76:4},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-143-6},
ISSN =	{1868-8969},
year =	{2020},
volume =	{164},
editor =	{Cabello, Sergio and Chen, Danny Z.},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2020.76},
URN =		{urn:nbn:de:0030-drops-122343},
doi =		{10.4230/LIPIcs.SoCG.2020.76},
annote =	{Keywords: weighted straight skeleton, implementation, visualization, graphical user interface, education}
}```
Document
##### Triconnected Planar Graphs of Maximum Degree Five are Subhamiltonian

Authors: Michael Hoffmann and Boris Klemz

Published in: LIPIcs, Volume 144, 27th Annual European Symposium on Algorithms (ESA 2019)

##### Abstract
We show that every triconnected planar graph of maximum degree five is subhamiltonian planar. A graph is subhamiltonian planar if it is a subgraph of a Hamiltonian planar graph or, equivalently, if it admits a 2-page book embedding. In fact, our result is stronger because we only require vertices of a separating triangle to have degree at most five, all other vertices may have arbitrary degree. This degree bound is tight: We describe a family of triconnected planar graphs that are not subhamiltonian planar and where every vertex of a separating triangle has degree at most six. Our results improve earlier work by Heath and by Bauernöppel and, independently, Bekos, Gronemann, and Raftopoulou, who showed that planar graphs of maximum degree three and four, respectively, are subhamiltonian planar. The proof is constructive and yields a quadratic time algorithm to obtain a subhamiltonian plane cycle for a given graph. As one of our main tools, which might be of independent interest, we devise an algorithm that, in a given 3-connected plane graph satisfying the above degree bounds, collapses each maximal separating triangle into a single edge such that the resulting graph is biconnected, contains no separating triangle, and no separation pair whose vertices are adjacent.

##### Cite as

Michael Hoffmann and Boris Klemz. Triconnected Planar Graphs of Maximum Degree Five are Subhamiltonian. In 27th Annual European Symposium on Algorithms (ESA 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 144, pp. 58:1-58:14, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2019)

```@InProceedings{hoffmann_et_al:LIPIcs.ESA.2019.58,
author =	{Hoffmann, Michael and Klemz, Boris},
title =	{{Triconnected Planar Graphs of Maximum Degree Five are Subhamiltonian}},
booktitle =	{27th Annual European Symposium on Algorithms (ESA 2019)},
pages =	{58:1--58:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-124-5},
ISSN =	{1868-8969},
year =	{2019},
volume =	{144},
editor =	{Bender, Michael A. and Svensson, Ola and Herman, Grzegorz},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2019.58},
URN =		{urn:nbn:de:0030-drops-111797},
doi =		{10.4230/LIPIcs.ESA.2019.58},
annote =	{Keywords: Graph drawing, book embedding, Hamiltonian graph, planar graph, bounded degree graph, graph augmentation, computational geometry, SPQR decomposition}
}```
Document
##### The Perfect Matching Reconfiguration Problem

Authors: Marthe Bonamy, Nicolas Bousquet, Marc Heinrich, Takehiro Ito, Yusuke Kobayashi, Arnaud Mary, Moritz Mühlenthaler, and Kunihiro Wasa

Published in: LIPIcs, Volume 138, 44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019)

##### Abstract
We study the perfect matching reconfiguration problem: Given two perfect matchings of a graph, is there a sequence of flip operations that transforms one into the other? Here, a flip operation exchanges the edges in an alternating cycle of length four. We are interested in the complexity of this decision problem from the viewpoint of graph classes. We first prove that the problem is PSPACE-complete even for split graphs and for bipartite graphs of bounded bandwidth with maximum degree five. We then investigate polynomial-time solvable cases. Specifically, we prove that the problem is solvable in polynomial time for strongly orderable graphs (that include interval graphs and strongly chordal graphs), for outerplanar graphs, and for cographs (also known as P_4-free graphs). Furthermore, for each yes-instance from these graph classes, we show that a linear number of flip operations is sufficient and we can exhibit a corresponding sequence of flip operations in polynomial time.

##### Cite as

Marthe Bonamy, Nicolas Bousquet, Marc Heinrich, Takehiro Ito, Yusuke Kobayashi, Arnaud Mary, Moritz Mühlenthaler, and Kunihiro Wasa. The Perfect Matching Reconfiguration Problem. In 44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 138, pp. 80:1-80:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

```@InProceedings{bonamy_et_al:LIPIcs.MFCS.2019.80,
author =	{Bonamy, Marthe and Bousquet, Nicolas and Heinrich, Marc and Ito, Takehiro and Kobayashi, Yusuke and Mary, Arnaud and M\"{u}hlenthaler, Moritz and Wasa, Kunihiro},
title =	{{The Perfect Matching Reconfiguration Problem}},
booktitle =	{44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019)},
pages =	{80:1--80:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-117-7},
ISSN =	{1868-8969},
year =	{2019},
volume =	{138},
editor =	{Rossmanith, Peter and Heggernes, Pinar and Katoen, Joost-Pieter},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2019.80},
URN =		{urn:nbn:de:0030-drops-110248},
doi =		{10.4230/LIPIcs.MFCS.2019.80},
annote =	{Keywords: Combinatorial Reconfiguration, Graph Algorithms, Perfect Matching}
}```
Document
##### On the Stretch Factor of Polygonal Chains

Authors: Ke Chen, Adrian Dumitrescu, Wolfgang Mulzer, and Csaba D. Tóth

Published in: LIPIcs, Volume 138, 44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019)

##### Abstract
Let P=(p_1, p_2, ..., p_n) be a polygonal chain. The stretch factor of P is the ratio between the total length of P and the distance of its endpoints, sum_{i = 1}^{n-1} |p_i p_{i+1}|/|p_1 p_n|. For a parameter c >= 1, we call P a c-chain if |p_ip_j|+|p_jp_k| <= c|p_ip_k|, for every triple (i,j,k), 1 <= i<j<k <= n. The stretch factor is a global property: it measures how close P is to a straight line, and it involves all the vertices of P; being a c-chain, on the other hand, is a fingerprint-property: it only depends on subsets of O(1) vertices of the chain. We investigate how the c-chain property influences the stretch factor in the plane: (i) we show that for every epsilon > 0, there is a noncrossing c-chain that has stretch factor Omega(n^{1/2-epsilon}), for sufficiently large constant c=c(epsilon); (ii) on the other hand, the stretch factor of a c-chain P is O(n^{1/2}), for every constant c >= 1, regardless of whether P is crossing or noncrossing; and (iii) we give a randomized algorithm that can determine, for a polygonal chain P in R^2 with n vertices, the minimum c >= 1 for which P is a c-chain in O(n^{2.5} polylog n) expected time and O(n log n) space.

##### Cite as

Ke Chen, Adrian Dumitrescu, Wolfgang Mulzer, and Csaba D. Tóth. On the Stretch Factor of Polygonal Chains. In 44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 138, pp. 56:1-56:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

```@InProceedings{chen_et_al:LIPIcs.MFCS.2019.56,
author =	{Chen, Ke and Dumitrescu, Adrian and Mulzer, Wolfgang and T\'{o}th, Csaba D.},
title =	{{On the Stretch Factor of Polygonal Chains}},
booktitle =	{44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019)},
pages =	{56:1--56:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-117-7},
ISSN =	{1868-8969},
year =	{2019},
volume =	{138},
editor =	{Rossmanith, Peter and Heggernes, Pinar and Katoen, Joost-Pieter},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2019.56},
URN =		{urn:nbn:de:0030-drops-110005},
doi =		{10.4230/LIPIcs.MFCS.2019.56},
annote =	{Keywords: polygonal chain, vertex dilation, Koch curve, recursive construction}
}```
Document
##### A Superlinear Lower Bound on the Number of 5-Holes

Authors: Oswin Aichholzer, Martin Balko, Thomas Hackl, Jan Kyncl, Irene Parada, Manfred Scheucher, Pavel Valtr, and Birgit Vogtenhuber

Published in: LIPIcs, Volume 77, 33rd International Symposium on Computational Geometry (SoCG 2017)

##### Abstract
Let P be a finite set of points in the plane in general position, that is, no three points of P are on a common line. We say that a set H of five points from P is a 5-hole in P if H is the vertex set of a convex 5-gon containing no other points of P. For a positive integer n, let h_5(n) be the minimum number of 5-holes among all sets of n points in the plane in general position. Despite many efforts in the last 30 years, the best known asymptotic lower and upper bounds for h_5(n) have been of order Omega(n) and O(n^2), respectively. We show that h_5(n) = Omega(n(log n)^(4/5)), obtaining the first superlinear lower bound on h_5(n). The following structural result, which might be of independent interest, is a crucial step in the proof of this lower bound. If a finite set P of points in the plane in general position is partitioned by a line l into two subsets, each of size at least 5 and not in convex position, then l intersects the convex hull of some 5-hole in P. The proof of this result is computer-assisted.

##### Cite as

Oswin Aichholzer, Martin Balko, Thomas Hackl, Jan Kyncl, Irene Parada, Manfred Scheucher, Pavel Valtr, and Birgit Vogtenhuber. A Superlinear Lower Bound on the Number of 5-Holes. In 33rd International Symposium on Computational Geometry (SoCG 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 77, pp. 8:1-8:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2017)

```@InProceedings{aichholzer_et_al:LIPIcs.SoCG.2017.8,
author =	{Aichholzer, Oswin and Balko, Martin and Hackl, Thomas and Kyncl, Jan and Parada, Irene and Scheucher, Manfred and Valtr, Pavel and Vogtenhuber, Birgit},
title =	{{A Superlinear Lower Bound on the Number of 5-Holes}},
booktitle =	{33rd International Symposium on Computational Geometry (SoCG 2017)},
pages =	{8:1--8:16},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-038-5},
ISSN =	{1868-8969},
year =	{2017},
volume =	{77},
editor =	{Aronov, Boris and Katz, Matthew J.},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2017.8},
URN =		{urn:nbn:de:0030-drops-72008},
doi =		{10.4230/LIPIcs.SoCG.2017.8},
annote =	{Keywords: Erd\"{o}s-Szekeres type problem, k-hole, empty k-gon, empty pentagon, planar point set}
}```
Document
##### Packing Short Plane Spanning Trees in Complete Geometric Graphs

Authors: Oswin Aichholzer, Thomas Hackl, Matias Korman, Alexander Pilz, Günter Rote, André van Renssen, Marcel Roeloffzen, and Birgit Vogtenhuber

Published in: LIPIcs, Volume 64, 27th International Symposium on Algorithms and Computation (ISAAC 2016)

##### Abstract
Given a set of points in the plane, we want to establish a connection network between these points that consists of several disjoint layers. Motivated by sensor networks, we want that each layer is spanning and plane, and that no edge is very long (when compared to the minimum length needed to obtain a spanning graph). We consider two different approaches: first we show an almost optimal centralized approach to extract two trees. Then we show a constant factor approximation for a distributed model in which each point can compute its adjacencies using only local information. This second approach may create cycles, but maintains planarity.

##### Cite as

Oswin Aichholzer, Thomas Hackl, Matias Korman, Alexander Pilz, Günter Rote, André van Renssen, Marcel Roeloffzen, and Birgit Vogtenhuber. Packing Short Plane Spanning Trees in Complete Geometric Graphs. In 27th International Symposium on Algorithms and Computation (ISAAC 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 64, pp. 9:1-9:12, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2016)

```@InProceedings{aichholzer_et_al:LIPIcs.ISAAC.2016.9,
author =	{Aichholzer, Oswin and Hackl, Thomas and Korman, Matias and Pilz, Alexander and Rote, G\"{u}nter and van Renssen, Andr\'{e} and Roeloffzen, Marcel and Vogtenhuber, Birgit},
title =	{{Packing Short Plane Spanning Trees in Complete Geometric Graphs}},
booktitle =	{27th International Symposium on Algorithms and Computation (ISAAC 2016)},
pages =	{9:1--9:12},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-026-2},
ISSN =	{1868-8969},
year =	{2016},
volume =	{64},
editor =	{Hong, Seok-Hee},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2016.9},
URN =		{urn:nbn:de:0030-drops-67823},
doi =		{10.4230/LIPIcs.ISAAC.2016.9},
annote =	{Keywords: Geometric Graphs, Graph Packing, Plane Graphs, Minimum Spanning Tree, Bottleneck Edge}
}```
Document
##### An Improved Lower Bound on the Minimum Number of Triangulations

Authors: Oswin Aichholzer, Victor Alvarez, Thomas Hackl, Alexander Pilz, Bettina Speckmann, and Birgit Vogtenhuber

Published in: LIPIcs, Volume 51, 32nd International Symposium on Computational Geometry (SoCG 2016)

##### Abstract
Upper and lower bounds for the number of geometric graphs of specific types on a given set of points in the plane have been intensively studied in recent years. For most classes of geometric graphs it is now known that point sets in convex position minimize their number. However, it is still unclear which point sets minimize the number of geometric triangulations; the so-called double circles are conjectured to be the minimizing sets. In this paper we prove that any set of n points in general position in the plane has at least Omega(2.631^n) geometric triangulations. Our result improves the previously best general lower bound of Omega(2.43^n) and also covers the previously best lower bound of Omega(2.63^n) for a fixed number of extreme points. We achieve our bound by showing and combining several new results, which are of independent interest: (1) Adding a point on the second convex layer of a given point set (of 7 or more points) at least doubles the number of triangulations. (2) Generalized configurations of points that minimize the number of triangulations have at most n/2 points on their convex hull. (3) We provide tight lower bounds for the number of triangulations of point sets with up to 15 points. These bounds further support the double circle conjecture.

##### Cite as

Oswin Aichholzer, Victor Alvarez, Thomas Hackl, Alexander Pilz, Bettina Speckmann, and Birgit Vogtenhuber. An Improved Lower Bound on the Minimum Number of Triangulations. In 32nd International Symposium on Computational Geometry (SoCG 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 51, pp. 7:1-7:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2016)

```@InProceedings{aichholzer_et_al:LIPIcs.SoCG.2016.7,
author =	{Aichholzer, Oswin and Alvarez, Victor and Hackl, Thomas and Pilz, Alexander and Speckmann, Bettina and Vogtenhuber, Birgit},
title =	{{An Improved Lower Bound on the Minimum Number of Triangulations}},
booktitle =	{32nd International Symposium on Computational Geometry (SoCG 2016)},
pages =	{7:1--7:16},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-009-5},
ISSN =	{1868-8969},
year =	{2016},
volume =	{51},
editor =	{Fekete, S\'{a}ndor and Lubiw, Anna},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2016.7},
URN =		{urn:nbn:de:0030-drops-58993},
doi =		{10.4230/LIPIcs.SoCG.2016.7},
annote =	{Keywords: Combinatorial geometry, Order types, Triangulations}
}```
Document
##### Order on Order Types

Authors: Alexander Pilz and Emo Welzl

Published in: LIPIcs, Volume 34, 31st International Symposium on Computational Geometry (SoCG 2015)

##### Abstract
Given P and P', equally sized planar point sets in general position, we call a bijection from P to P' crossing-preserving if crossings of connecting segments in P are preserved in P' (extra crossings may occur in P'). If such a mapping exists, we say that P' crossing-dominates P, and if such a mapping exists in both directions, P and P' are called crossing-equivalent. The relation is transitive, and we have a partial order on the obtained equivalence classes (called crossing types or x-types). Point sets of equal order type are clearly crossing-equivalent, but not vice versa. Thus, x-types are a coarser classification than order types. (We will see, though, that a collapse of different order types to one x-type occurs for sets with triangular convex hull only.) We argue that either the maximal or the minimal x-types are sufficient for answering many combinatorial (existential or extremal) questions on planar point sets. Motivated by this we consider basic properties of the relation. We characterize order types crossing-dominated by points in convex position. Further, we give a full characterization of minimal and maximal abstract order types. Based on that, we provide a polynomial-time algorithm to check whether a point set crossing-dominates another. Moreover, we generate all maximal and minimal x-types for small numbers of points.

##### Cite as

Alexander Pilz and Emo Welzl. Order on Order Types. In 31st International Symposium on Computational Geometry (SoCG 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 34, pp. 285-299, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

```@InProceedings{pilz_et_al:LIPIcs.SOCG.2015.285,
author =	{Pilz, Alexander and Welzl, Emo},
title =	{{Order on Order Types}},
booktitle =	{31st International Symposium on Computational Geometry (SoCG 2015)},
pages =	{285--299},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-939897-83-5},
ISSN =	{1868-8969},
year =	{2015},
volume =	{34},
editor =	{Arge, Lars and Pach, J\'{a}nos},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SOCG.2015.285},
URN =		{urn:nbn:de:0030-drops-51194},
doi =		{10.4230/LIPIcs.SOCG.2015.285},
annote =	{Keywords: point set, order type, planar graph, crossing-free geometric graph}
}```
Document
##### Arc Diagrams, Flip Distances, and Hamiltonian Triangulations

Authors: Jean Cardinal, Michael Hoffmann, Vincent Kusters, Csaba D. Tóth, and Manuel Wettstein

Published in: LIPIcs, Volume 30, 32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015)

##### Abstract
We show that every triangulation (maximal planar graph) on n\ge 6 vertices can be flipped into a Hamiltonian triangulation using a sequence of less than n/2 combinatorial edge flips. The previously best upper bound uses 4-connectivity as a means to establish Hamiltonicity. But in general about 3n/5 flips are necessary to reach a 4-connected triangulation. Our result improves the upper bound on the diameter of the flip graph of combinatorial triangulations on n vertices from 5.2n-33.6 to 5n-23. We also show that for every triangulation on n vertices there is a simultaneous flip of less than 2n/3 edges to a 4-connected triangulation. The bound on the number of edges is tight, up to an additive constant. As another application we show that every planar graph on n vertices admits an arc diagram with less than n/2 biarcs, that is, after subdividing less than n/2 (of potentially 3n-6) edges the resulting graph admits a 2-page book embedding.

##### Cite as

Jean Cardinal, Michael Hoffmann, Vincent Kusters, Csaba D. Tóth, and Manuel Wettstein. Arc Diagrams, Flip Distances, and Hamiltonian Triangulations. In 32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 30, pp. 197-210, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

```@InProceedings{cardinal_et_al:LIPIcs.STACS.2015.197,
author =	{Cardinal, Jean and Hoffmann, Michael and Kusters, Vincent and T\'{o}th, Csaba D. and Wettstein, Manuel},
title =	{{Arc Diagrams, Flip Distances, and Hamiltonian Triangulations}},
booktitle =	{32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015)},
pages =	{197--210},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-939897-78-1},
ISSN =	{1868-8969},
year =	{2015},
volume =	{30},
editor =	{Mayr, Ernst W. and Ollinger, Nicolas},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2015.197},
URN =		{urn:nbn:de:0030-drops-49141},
doi =		{10.4230/LIPIcs.STACS.2015.197},
annote =	{Keywords: graph embeddings, edge flips, flip graph, separating triangles}
}```
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