{"title":"Delaunay and Regular Triangulations as Lexicographic Optimal Chains","authors":"D. Cohen-Steiner, A. Lieutier, J. Vuillamy","doi":"10.1007/s00454-023-00485-1","DOIUrl":"https://doi.org/10.1007/s00454-023-00485-1","url":null,"abstract":"","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"70 1","pages":"1 - 50"},"PeriodicalIF":0.8,"publicationDate":"2023-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45486294","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Tropical Carathéodory with Matroids.","authors":"Georg Loho, Raman Sanyal","doi":"10.1007/s00454-022-00446-0","DOIUrl":"https://doi.org/10.1007/s00454-022-00446-0","url":null,"abstract":"<p><p>Bárány's colorful generalization of Carathéodory's Theorem combines geometrical and combinatorial constraints. Kalai-Meshulam (2005) and Holmsen (2016) generalized Bárány's theorem by replacing color classes with matroid constraints. In this note, we obtain corresponding results in tropical convexity, generalizing the Tropical Colorful Carathéodory Theorem of Gaubert-Meunier (2010). Our proof is inspired by geometric arguments and is reminiscent of matroid intersection. Moreover, we show that the topological approach fails in this setting. We also discuss tropical colorful linear programming and show that it is NP-complete. We end with thoughts and questions on generalizations to polymatroids, anti-matroids as well as examples and matroid simplicial depth.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"69 1","pages":"139-155"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9805987/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"10494381","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Katrin Casel, Henning Fernau, Alexander Grigoriev, Markus L Schmid, Sue Whitesides
{"title":"Combinatorial Properties and Recognition of Unit Square Visibility Graphs.","authors":"Katrin Casel, Henning Fernau, Alexander Grigoriev, Markus L Schmid, Sue Whitesides","doi":"10.1007/s00454-022-00414-8","DOIUrl":"10.1007/s00454-022-00414-8","url":null,"abstract":"<p><p>Unit square visibility graphs (USV) are described by axis-parallel visibility between unit squares placed in the plane. If the squares are required to be placed on integer grid coordinates, then USV become unit square grid visibility graphs (USGV), an alternative characterisation of the well-known rectilinear graphs. We extend known combinatorial results for USGV and we show that, in the weak case (i.e., visibilities do not necessarily translate into edges of the represented combinatorial graph), the area minimisation variant of their recognition problem is <math><mrow><mspace></mspace><mrow><mi>N</mi><mi>P</mi></mrow><mspace></mspace></mrow></math>-hard. We also provide combinatorial insights with respect to USV, and as our main result, we prove their recognition problem to be <math><mrow><mspace></mspace><mrow><mi>N</mi><mi>P</mi></mrow><mspace></mspace></mrow></math>-hard, which settles an open question.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"69 4","pages":"937-980"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10169907/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"10301082","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Maximum Matchings in Geometric Intersection Graphs.","authors":"Édouard Bonnet, Sergio Cabello, Wolfgang Mulzer","doi":"10.1007/s00454-023-00564-3","DOIUrl":"10.1007/s00454-023-00564-3","url":null,"abstract":"<p><p>Let <i>G</i> be an intersection graph of <i>n</i> geometric objects in the plane. We show that a maximum matching in <i>G</i> can be found in <math><mrow><mi>O</mi><mspace></mspace><mo>(</mo><msup><mi>ρ</mi><mrow><mn>3</mn><mi>ω</mi><mo>/</mo><mn>2</mn></mrow></msup><msup><mi>n</mi><mrow><mi>ω</mi><mo>/</mo><mn>2</mn></mrow></msup><mo>)</mo></mrow></math> time with high probability, where <math><mi>ρ</mi></math> is the density of the geometric objects and <math><mrow><mi>ω</mi><mo>></mo><mn>2</mn></mrow></math> is a constant such that <math><mrow><mi>n</mi><mo>×</mo><mi>n</mi></mrow></math> matrices can be multiplied in <math><mrow><mi>O</mi><mo>(</mo><msup><mi>n</mi><mi>ω</mi></msup><mo>)</mo></mrow></math> time. The same result holds for any subgraph of <i>G</i>, as long as a geometric representation is at hand. For this, we combine algebraic methods, namely computing the rank of a matrix via Gaussian elimination, with the fact that geometric intersection graphs have small separators. We also show that in many interesting cases, the maximum matching problem in a general geometric intersection graph can be reduced to the case of bounded density. In particular, a maximum matching in the intersection graph of any family of translates of a convex object in the plane can be found in <math><mrow><mi>O</mi><mo>(</mo><msup><mi>n</mi><mrow><mi>ω</mi><mo>/</mo><mn>2</mn></mrow></msup><mo>)</mo></mrow></math> time with high probability, and a maximum matching in the intersection graph of a family of planar disks with radii in <math><mrow><mo>[</mo><mn>1</mn><mo>,</mo><mi>Ψ</mi><mo>]</mo></mrow></math> can be found in <math><mrow><mi>O</mi><mspace></mspace><mo>(</mo><msup><mi>Ψ</mi><mn>6</mn></msup><msup><mo>log</mo><mn>11</mn></msup><mspace></mspace><mi>n</mi><mo>+</mo><msup><mi>Ψ</mi><mrow><mn>12</mn><mi>ω</mi></mrow></msup><msup><mi>n</mi><mrow><mi>ω</mi><mo>/</mo><mn>2</mn></mrow></msup><mo>)</mo></mrow></math> time with high probability.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"70 3","pages":"550-579"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10550895/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41156223","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Nearly <i>k</i>-Distance Sets.","authors":"Nóra Frankl, Andrey Kupavskii","doi":"10.1007/s00454-023-00489-x","DOIUrl":"https://doi.org/10.1007/s00454-023-00489-x","url":null,"abstract":"<p><p>We say that a set of points <math><mrow><mi>S</mi><mo>⊂</mo><msup><mrow><mi>R</mi></mrow><mi>d</mi></msup></mrow></math> is an <math><mi>ε</mi></math>-nearly <i>k</i>-distance set if there exist <math><mrow><mn>1</mn><mo>≤</mo><msub><mi>t</mi><mn>1</mn></msub><mo>≤</mo><mo>…</mo><mo>≤</mo><msub><mi>t</mi><mi>k</mi></msub></mrow></math>, such that the distance between any two distinct points in <i>S</i> falls into <math><mrow><mrow><mo>[</mo><msub><mi>t</mi><mn>1</mn></msub><mo>,</mo><msub><mi>t</mi><mn>1</mn></msub><mo>+</mo><mi>ε</mi><mo>]</mo></mrow><mo>∪</mo><mo>⋯</mo><mo>∪</mo><mrow><mo>[</mo><msub><mi>t</mi><mi>k</mi></msub><mo>,</mo><msub><mi>t</mi><mi>k</mi></msub><mo>+</mo><mi>ε</mi><mo>]</mo></mrow></mrow></math>. In this paper, we study the quantity <dispformula><math><mrow><mtable><mtr><mtd><mrow><msub><mi>M</mi><mi>k</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>)</mo></mrow><mo>=</mo><munder><mo>lim</mo><mrow><mi>ε</mi><mo>→</mo><mn>0</mn></mrow></munder><mo>max</mo><mrow><mo>{</mo><mo>|</mo><mi>S</mi><mo>|</mo><mo>:</mo><mi>S</mi><mspace></mspace><mspace></mspace><mtext>is an</mtext><mspace></mspace><mi>ε</mi><mtext>-nearly</mtext><mspace></mspace><mi>k</mi><mtext>-distance set in</mtext><mspace></mspace><msup><mrow><mi>R</mi></mrow><mi>d</mi></msup><mo>}</mo></mrow></mrow></mtd></mtr></mtable></mrow></math></dispformula>and its relation to the classical quantity <math><mrow><msub><mi>m</mi><mi>k</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>)</mo></mrow></mrow></math>: the size of the largest <i>k</i>-distance set in <math><msup><mrow><mi>R</mi></mrow><mi>d</mi></msup></math>. We obtain that <math><mrow><msub><mi>M</mi><mi>k</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>)</mo></mrow><mo>=</mo><msub><mi>m</mi><mi>k</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>)</mo></mrow></mrow></math> for <math><mrow><mi>k</mi><mo>=</mo><mn>2</mn><mo>,</mo><mn>3</mn></mrow></math>, as well as for any fixed <i>k</i>, provided that <i>d</i> is sufficiently large. The last result answers a question, proposed by Erdős, Makai, and Pach. We also address a closely related Turán-type problem, studied by Erdős, Makai, Pach, and Spencer in the 90s: given <i>n</i> points in <math><msup><mrow><mi>R</mi></mrow><mi>d</mi></msup></math>, how many pairs of them form a distance that belongs to <math><mrow><mrow><mo>[</mo><msub><mi>t</mi><mn>1</mn></msub><mo>,</mo><msub><mi>t</mi><mn>1</mn></msub><mo>+</mo><mn>1</mn><mo>]</mo></mrow><mo>∪</mo><mo>⋯</mo><mo>∪</mo><mrow><mo>[</mo><msub><mi>t</mi><mi>k</mi></msub><mo>,</mo><msub><mi>t</mi><mi>k</mi></msub><mo>+</mo><mn>1</mn><mo>]</mo></mrow></mrow></math>, where <math><mrow><msub><mi>t</mi><mn>1</mn></msub><mo>,</mo><mo>⋯</mo><mo>,</mo><msub><mi>t</mi><mi>k</mi></msub></mrow></math> are fixed and any two points in the set are at distance at least 1 apart? We establish the connection between this quantity and a quantity closely related to <math><mrow><msub><mi>M</mi><mi>k</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>-</mo><mn>1</mn><m","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"70 3","pages":"455-494"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10550902/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41158582","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Lonely Points in Simplices.","authors":"Maximilian Jaroschek, Manuel Kauers, Laura Kovács","doi":"10.1007/s00454-022-00428-2","DOIUrl":"10.1007/s00454-022-00428-2","url":null,"abstract":"<p><p>Given a lattice <math><mrow><mi>L</mi> <mo>⊆</mo> <msup><mi>Z</mi> <mi>m</mi></msup> </mrow> </math> and a subset <math><mrow><mi>A</mi> <mo>⊆</mo> <msup><mi>R</mi> <mi>m</mi></msup> </mrow> </math> , we say that a point in <i>A</i> is <i>lonely</i> if it is not equivalent modulo <math><mi>L</mi></math> to another point of <i>A</i>. We are interested in identifying lonely points for specific choices of <math><mi>L</mi></math> when <i>A</i> is a dilated standard simplex, and in conditions on <math><mi>L</mi></math> which ensure that the number of lonely points is unbounded as the simplex dilation goes to infinity.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"69 1","pages":"4-25"},"PeriodicalIF":0.6,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9805990/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"10481510","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Cusp Density and Commensurability of Non-arithmetic Hyperbolic Coxeter Orbifolds.","authors":"Edoardo Dotti, Simon T Drewitz, Ruth Kellerhals","doi":"10.1007/s00454-022-00455-z","DOIUrl":"https://doi.org/10.1007/s00454-022-00455-z","url":null,"abstract":"<p><p>For three distinct infinite families <math><mrow><mo>(</mo> <msub><mi>R</mi> <mi>m</mi></msub> <mo>)</mo></mrow> </math> , <math><mrow><mo>(</mo> <msub><mi>S</mi> <mi>m</mi></msub> <mo>)</mo></mrow> </math> , and <math><mrow><mo>(</mo> <msub><mi>T</mi> <mi>m</mi></msub> <mo>)</mo></mrow> </math> of non-arithmetic 1-cusped hyperbolic Coxeter 3-orbifolds, we prove incommensurability for a pair of elements <math><msub><mi>X</mi> <mi>k</mi></msub> </math> and <math><msub><mi>Y</mi> <mi>l</mi></msub> </math> belonging to the same sequence and for most pairs belonging two different ones. We investigate this problem first by means of the Vinberg space and the Vinberg form, a quadratic space associated to each of the corresponding fundamental Coxeter prism groups, which allows us to deduce some partial results. The complete proof is based on the analytic behavior of another commensurability invariant. It is given by the cusp density, and we prove and exploit its strict monotonicity.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"69 3","pages":"873-895"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9984359/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"9424995","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Deletion in Abstract Voronoi Diagrams in Expected Linear Time and Related Problems.","authors":"Kolja Junginger, Evanthia Papadopoulou","doi":"10.1007/s00454-022-00463-z","DOIUrl":"10.1007/s00454-022-00463-z","url":null,"abstract":"<p><p>Updating an abstract Voronoi diagram in linear time, after deletion of one site, has been an open problem in a long time; similarly, for any concrete Voronoi diagram of generalized (non-point) sites. In this paper we present a simple, expected linear-time algorithm to update an abstract Voronoi diagram after deletion of one site. To achieve this result, we use the concept of a Voronoi-like diagram, a relaxed Voronoi structure of independent interest. Voronoi-like diagrams serve as intermediate structures, which are considerably simpler to compute, thus, making an expected linear-time construction possible. We formalize the concept and prove that it is robust under insertion, therefore, enabling its use in incremental constructions. The time-complexity analysis introduces a variant to backwards analysis, which is applicable to order-dependent structures. We further extend the technique to compute in expected linear time: the order-<math><mrow><mo>(</mo><mi>k</mi><mspace></mspace><mo>+</mo><mspace></mspace><mn>1</mn><mo>)</mo></mrow></math> subdivision within an order-<i>k</i> Voronoi region, and the farthest abstract Voronoi diagram, after the order of its regions at infinity is known.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"69 4","pages":"1040-1078"},"PeriodicalIF":0.8,"publicationDate":"2023-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10169906/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"10296942","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}