# 8 Search Results for "Kyncl, Jan"

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
##### Barycentric Cuts Through a Convex Body

Authors: Zuzana Patáková, Martin Tancer, and Uli Wagner

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

##### Abstract
Let K be a convex body in ℝⁿ (i.e., a compact convex set with nonempty interior). Given a point p in the interior of K, a hyperplane h passing through p is called barycentric if p is the barycenter of K ∩ h. In 1961, Grünbaum raised the question whether, for every K, there exists an interior point p through which there are at least n+1 distinct barycentric hyperplanes. Two years later, this was seemingly resolved affirmatively by showing that this is the case if p=p₀ is the point of maximal depth in K. However, while working on a related question, we noticed that one of the auxiliary claims in the proof is incorrect. Here, we provide a counterexample; this re-opens Grünbaum’s question. It follows from known results that for n ≥ 2, there are always at least three distinct barycentric cuts through the point p₀ ∈ K of maximal depth. Using tools related to Morse theory we are able to improve this bound: four distinct barycentric cuts through p₀ are guaranteed if n ≥ 3.

##### Cite as

Zuzana Patáková, Martin Tancer, and Uli Wagner. Barycentric Cuts Through a Convex Body. In 36th International Symposium on Computational Geometry (SoCG 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 164, pp. 62:1-62:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2020)

```@InProceedings{patakova_et_al:LIPIcs.SoCG.2020.62,
author =	{Pat\'{a}kov\'{a}, Zuzana and Tancer, Martin and Wagner, Uli},
title =	{{Barycentric Cuts Through a Convex Body}},
booktitle =	{36th International Symposium on Computational Geometry (SoCG 2020)},
pages =	{62:1--62:16},
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.2020.62},
URN =		{urn:nbn:de:0030-drops-122201},
doi =		{10.4230/LIPIcs.SoCG.2020.62},
annote =	{Keywords: convex body, barycenter, Tukey depth, smooth manifold, critical points}
}```
Document
##### Z_2-Genus of Graphs and Minimum Rank of Partial Symmetric Matrices

Authors: Radoslav Fulek and Jan Kynčl

Published in: LIPIcs, Volume 129, 35th International Symposium on Computational Geometry (SoCG 2019)

##### Abstract
The genus g(G) of a graph G is the minimum g such that G has an embedding on the orientable surface M_g of genus g. A drawing of a graph on a surface is independently even if every pair of nonadjacent edges in the drawing crosses an even number of times. The Z_2-genus of a graph G, denoted by g_0(G), is the minimum g such that G has an independently even drawing on M_g. By a result of Battle, Harary, Kodama and Youngs from 1962, the graph genus is additive over 2-connected blocks. In 2013, Schaefer and Štefankovič proved that the Z_2-genus of a graph is additive over 2-connected blocks as well, and asked whether this result can be extended to so-called 2-amalgamations, as an analogue of results by Decker, Glover, Huneke, and Stahl for the genus. We give the following partial answer. If G=G_1 cup G_2, G_1 and G_2 intersect in two vertices u and v, and G-u-v has k connected components (among which we count the edge uv if present), then |g_0(G)-(g_0(G_1)+g_0(G_2))|<=k+1. For complete bipartite graphs K_{m,n}, with n >= m >= 3, we prove that g_0(K_{m,n})/g(K_{m,n})=1-O(1/n). Similar results are proved also for the Euler Z_2-genus. We express the Z_2-genus of a graph using the minimum rank of partial symmetric matrices over Z_2; a problem that might be of independent interest.

##### Cite as

Radoslav Fulek and Jan Kynčl. Z_2-Genus of Graphs and Minimum Rank of Partial Symmetric Matrices. In 35th International Symposium on Computational Geometry (SoCG 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 129, pp. 39:1-39:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2019)

```@InProceedings{fulek_et_al:LIPIcs.SoCG.2019.39,
author =	{Fulek, Radoslav and Kyn\v{c}l, Jan},
title =	{{Z\underline2-Genus of Graphs and Minimum Rank of Partial Symmetric Matrices}},
booktitle =	{35th International Symposium on Computational Geometry (SoCG 2019)},
pages =	{39:1--39:16},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-104-7},
ISSN =	{1868-8969},
year =	{2019},
volume =	{129},
editor =	{Barequet, Gill and Wang, Yusu},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2019.39},
URN =		{urn:nbn:de:0030-drops-104439},
doi =		{10.4230/LIPIcs.SoCG.2019.39},
annote =	{Keywords: graph genus, minimum rank of a partial matrix, Hanani-Tutte theorem, graph amalgamation}
}```
Document
##### Hanani-Tutte for Approximating Maps of Graphs

Authors: Radoslav Fulek and Jan Kyncl

Published in: LIPIcs, Volume 99, 34th International Symposium on Computational Geometry (SoCG 2018)

##### Abstract
We resolve in the affirmative conjectures of A. Skopenkov and Repovs (1998), and M. Skopenkov (2003) generalizing the classical Hanani-Tutte theorem to the setting of approximating maps of graphs on 2-dimensional surfaces by embeddings. Our proof of this result is constructive and almost immediately implies an efficient algorithm for testing whether a given piecewise linear map of a graph in a surface is approximable by an embedding. More precisely, an instance of this problem consists of (i) a graph G whose vertices are partitioned into clusters and whose inter-cluster edges are partitioned into bundles, and (ii) a region R of a 2-dimensional compact surface M given as the union of a set of pairwise disjoint discs corresponding to the clusters and a set of pairwise disjoint "pipes" corresponding to the bundles, connecting certain pairs of these discs. We are to decide whether G can be embedded inside M so that the vertices in every cluster are drawn in the corresponding disc, the edges in every bundle pass only through its corresponding pipe, and every edge crosses the boundary of each disc at most once.

##### Cite as

Radoslav Fulek and Jan Kyncl. Hanani-Tutte for Approximating Maps of Graphs. In 34th International Symposium on Computational Geometry (SoCG 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 99, pp. 39:1-39:15, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)

```@InProceedings{fulek_et_al:LIPIcs.SoCG.2018.39,
author =	{Fulek, Radoslav and Kyncl, Jan},
title =	{{Hanani-Tutte for Approximating Maps of Graphs}},
booktitle =	{34th International Symposium on Computational Geometry (SoCG 2018)},
pages =	{39:1--39:15},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-066-8},
ISSN =	{1868-8969},
year =	{2018},
volume =	{99},
editor =	{Speckmann, Bettina and T\'{o}th, Csaba D.},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2018.39},
URN =		{urn:nbn:de:0030-drops-87527},
doi =		{10.4230/LIPIcs.SoCG.2018.39},
annote =	{Keywords: Hanani-Tutte theorem, graph embedding, map approximation, weak embedding, clustered planarity}
}```
Document
##### The Z_2-Genus of Kuratowski Minors

Authors: Radoslav Fulek and Jan Kyncl

Published in: LIPIcs, Volume 99, 34th International Symposium on Computational Geometry (SoCG 2018)

##### Abstract
A drawing of a graph on a surface is independently even if every pair of nonadjacent edges in the drawing crosses an even number of times. The Z_2-genus of a graph G is the minimum g such that G has an independently even drawing on the orientable surface of genus g. An unpublished result by Robertson and Seymour implies that for every t, every graph of sufficiently large genus contains as a minor a projective t x t grid or one of the following so-called t-Kuratowski graphs: K_{3,t}, or t copies of K_5 or K_{3,3} sharing at most 2 common vertices. We show that the Z_2-genus of graphs in these families is unbounded in t; in fact, equal to their genus. Together, this implies that the genus of a graph is bounded from above by a function of its Z_2-genus, solving a problem posed by Schaefer and Stefankovic, and giving an approximate version of the Hanani-Tutte theorem on orientable surfaces.

##### Cite as

Radoslav Fulek and Jan Kyncl. The Z_2-Genus of Kuratowski Minors. In 34th International Symposium on Computational Geometry (SoCG 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 99, pp. 40:1-40:14, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)

```@InProceedings{fulek_et_al:LIPIcs.SoCG.2018.40,
author =	{Fulek, Radoslav and Kyncl, Jan},
title =	{{The Z\underline2-Genus of Kuratowski Minors}},
booktitle =	{34th International Symposium on Computational Geometry (SoCG 2018)},
pages =	{40:1--40:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-066-8},
ISSN =	{1868-8969},
year =	{2018},
volume =	{99},
editor =	{Speckmann, Bettina and T\'{o}th, Csaba D.},
publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2018.40},
URN =		{urn:nbn:de:0030-drops-87539},
doi =		{10.4230/LIPIcs.SoCG.2018.40},
annote =	{Keywords: Hanani-Tutte theorem, genus of a graph, Z\underline2-genus of a graph, Kuratowski graph}
}```
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
##### 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
##### Recognition and Complexity of Point Visibility Graphs

Authors: Jean Cardinal and Udo Hoffmann

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

##### Abstract
A point visibility graph is a graph induced by a set of points in the plane, where every vertex corresponds to a point, and two vertices are adjacent whenever the two corresponding points are visible from each other, that is, the open segment between them does not contain any other point of the set. We study the recognition problem for point visibility graphs: given a simple undirected graph, decide whether it is the visibility graph of some point set in the plane. We show that the problem is complete for the existential theory of the reals. Hence the problem is as hard as deciding the existence of a real solution to a system of polynomial inequalities. The proof involves simple substructures forcing collinearities in all realizations of some visibility graphs, which are applied to the algebraic universality constructions of Mnev and Richter-Gebert. This solves a longstanding open question and paves the way for the analysis of other classes of visibility graphs. Furthermore, as a corollary of one of our construction, we show that there exist point visibility graphs that do not admit any geometric realization with points having integer coordinates.

##### Cite as

Jean Cardinal and Udo Hoffmann. Recognition and Complexity of Point Visibility Graphs. In 31st International Symposium on Computational Geometry (SoCG 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 34, pp. 171-185, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

```@InProceedings{cardinal_et_al:LIPIcs.SOCG.2015.171,
author =	{Cardinal, Jean and Hoffmann, Udo},
title =	{{Recognition and Complexity of Point Visibility Graphs}},
booktitle =	{31st International Symposium on Computational Geometry (SoCG 2015)},
pages =	{171--185},
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.2015.171},
URN =		{urn:nbn:de:0030-drops-51390},
doi =		{10.4230/LIPIcs.SOCG.2015.171},
annote =	{Keywords: point visibility graphs, recognition, existential theory of the reals}
}```
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