Pub Date : 2023-10-01DOI: 10.1016/j.comgeo.2023.102011
Ahmad Biniaz , Prosenjit Bose , Yunkai Wang
A set of disks in the plane is said to be pierced by a point set P if each disk in contains a point of P. Any set of pairwise intersecting unit disks can be pierced by 3 points (Hadwiger and Debrunner (1955) [7]). Stachó and independently Danzer established that any set of pairwise intersecting arbitrary disks can be pierced by 4 points (Stachó (1981–1984) [16]. Danzer (1986) [4]). Existing linear-time algorithms for finding a set of 4 or 5 points that pierce pairwise intersecting disks of arbitrary radius use the LP-type problem as a subroutine. We present simple linear-time algorithms for finding 3 points for piercing pairwise intersecting unit disks, and 5 points for piercing pairwise intersecting disks of arbitrary radius. Our algorithms use simple geometric transformations and avoid heavy machinery. We also show that 3 points are sometimes necessary for piercing pairwise intersecting unit disks.
{"title":"Simple linear time algorithms for piercing pairwise intersecting disks","authors":"Ahmad Biniaz , Prosenjit Bose , Yunkai Wang","doi":"10.1016/j.comgeo.2023.102011","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102011","url":null,"abstract":"<div><p>A set <span><math><mi>D</mi></math></span> of disks in the plane is said to be pierced by a point set <em>P</em> if each disk in <span><math><mi>D</mi></math></span> contains a point of <em>P</em>. Any set of pairwise intersecting unit disks can be pierced by 3 points (Hadwiger and Debrunner (1955) <span>[7]</span>). Stachó and independently Danzer established that any set of pairwise intersecting arbitrary disks can be pierced by 4 points (Stachó (1981–1984) <span>[16]</span>. Danzer (1986) <span>[4]</span><span>). Existing linear-time algorithms for finding a set of 4 or 5 points that pierce pairwise intersecting disks of arbitrary radius use the LP-type problem as a subroutine. We present simple linear-time algorithms for finding 3 points for piercing pairwise intersecting unit disks, and 5 points for piercing pairwise intersecting disks of arbitrary radius. Our algorithms use simple geometric transformations and avoid heavy machinery. We also show that 3 points are sometimes necessary for piercing pairwise intersecting unit disks.</span></p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49790335","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-10-01DOI: 10.1016/j.comgeo.2023.102010
Joseph O'Rourke , Costin Vîlcu
We prove that every positively weighted tree T can be realized as the cut locus of a point x on a convex polyhedron P, with T edge weights matching edge lengths. If T has n leaves, P has (in general) vertices. We show there is in fact a continuum of polyhedra P each realizing T for some . Three main tools in the proof are properties of the star unfolding of P, Alexandrov's gluing theorem, and a new cut-locus partition lemma. The construction of P from T is surprisingly simple.
{"title":"Cut locus realizations on convex polyhedra","authors":"Joseph O'Rourke , Costin Vîlcu","doi":"10.1016/j.comgeo.2023.102010","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102010","url":null,"abstract":"<div><p>We prove that every positively weighted tree <em>T</em> can be realized as the cut locus <span><math><mi>C</mi><mo>(</mo><mi>x</mi><mo>)</mo></math></span> of a point <em>x</em><span> on a convex polyhedron </span><em>P</em>, with <em>T</em> edge weights matching <span><math><mi>C</mi><mo>(</mo><mi>x</mi><mo>)</mo></math></span> edge lengths. If <em>T</em> has <em>n</em> leaves, <em>P</em> has (in general) <span><math><mi>n</mi><mo>+</mo><mn>1</mn></math></span><span> vertices. We show there is in fact a continuum of polyhedra </span><em>P</em> each realizing <em>T</em> for some <span><math><mi>x</mi><mo>∈</mo><mi>P</mi></math></span>. Three main tools in the proof are properties of the star unfolding of <em>P</em>, Alexandrov's gluing theorem, and a new cut-locus partition lemma. The construction of <em>P</em> from <em>T</em> is surprisingly simple.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49790334","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-10-01DOI: 10.1016/j.comgeo.2023.102019
Pankaj K. Agarwal , Tzvika Geft , Dan Halperin , Erin Taylor
We study the problem of motion planning for a collection of n labeled unit disc robots in a polygonal environment. We assume that the robots have revolving areas around their start and final positions: that each start and each final is contained in a radius 2 disc lying in the free space, not necessarily concentric with the start or final position, which is free from other start or final positions. This assumption allows a weakly-monotone motion plan, in which robots move according to an ordering as follows: during the turn of a robot R in the ordering, it moves fully from its start to final position, while other robots do not leave their revolving areas. As R passes through a revolving area, a robot that is inside this area may move within the revolving area to avoid a collision. Notwithstanding the existence of a motion plan, we show that minimizing the total traveled distance in this setting, specifically even when the motion plan is restricted to be weakly-monotone, is APX-hard, ruling out any polynomial-time -approximation algorithm.
On the positive side, we present the first constant-factor approximation algorithm for computing a feasible weakly-monotone motion plan. The total distance traveled by the robots is within an factor of that of the optimal motion plan, which need not be weakly monotone. Our algorithm extends to an online setting in which the polygonal environment is fixed but the initial and final positions of robots are specified in an online manner. Finally, we observe that the overhead in the overall cost that we add while editing the paths to avoid robot-robot collision can vary significantly depending on the ordering we chose. Finding the best ordering in this respect is known to be NP-hard, and we provide a polynomial time -approximation algorithm for this problem.
{"title":"Multi-robot motion planning for unit discs with revolving areas","authors":"Pankaj K. Agarwal , Tzvika Geft , Dan Halperin , Erin Taylor","doi":"10.1016/j.comgeo.2023.102019","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102019","url":null,"abstract":"<div><p>We study the problem of motion planning for a collection of <em>n</em> labeled unit disc robots in a polygonal environment. We assume that the robots have <em>revolving areas</em> around their start and final positions: that each start and each final is contained in a radius 2 disc lying in the free space, not necessarily concentric with the start or final position, which is free from other start or final positions. This assumption allows a <em>weakly-monotone</em> motion plan, in which robots move according to an ordering as follows: during the turn of a robot <em>R</em> in the ordering, it moves fully from its start to final position, while other robots do not leave their revolving areas. As <em>R</em> passes through a revolving area, a robot <span><math><msup><mrow><mi>R</mi></mrow><mrow><mo>′</mo></mrow></msup></math></span> that is inside this area may move within the revolving area to avoid a collision. Notwithstanding the existence of a motion plan, we show that minimizing the total traveled distance in this setting, specifically even when the motion plan is restricted to be weakly-monotone, is APX-hard, ruling out any polynomial-time <span><math><mo>(</mo><mn>1</mn><mo>+</mo><mi>ε</mi><mo>)</mo></math></span>-approximation algorithm.</p><p><span>On the positive side, we present the first constant-factor approximation algorithm for computing a feasible weakly-monotone motion plan. The total distance traveled by the robots is within an </span><span><math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math></span><span> factor of that of the optimal motion plan, which need not be weakly monotone. Our algorithm extends to an online setting in which the polygonal environment is fixed but the initial and final positions of robots are specified in an online manner. Finally, we observe that the overhead in the overall cost that we add while editing the paths to avoid robot-robot collision can vary significantly depending on the ordering we chose. Finding the best ordering in this respect is known to be NP-hard, and we provide a polynomial time </span><span><math><mi>O</mi><mo>(</mo><mi>log</mi><mo></mo><mi>n</mi><mi>log</mi><mo></mo><mi>log</mi><mo></mo><mi>n</mi><mo>)</mo></math></span>-approximation algorithm for this problem.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49830655","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-09-11DOI: 10.1016/j.comgeo.2023.102053
Pankaj K. Agarwal , Matthew J. Katz , Micha Sharir
Let S be a set of n geometric objects of constant complexity (e.g., points, line segments, disks, ellipses) in , and let be a distance function on S. For a parameter , we define the proximity graph where . Given S, , and an integer , the reverse-shortest-path (RSP) problem asks for computing the smallest value such that contains a path from s to t of length at most k.
In this paper we present a general randomized technique that solves the RSP problem efficiently for a large family of geometric objects and distance functions. Using standard, and sometimes more involved, semi-algebraic range-searching techniques, we first give an efficient algorithm for the decision problem, namely, given a value , determine whether contains a path from s to t of length at most k. Next, we adapt our decision algorithm and combine it with a random-sampling method to compute , by efficiently performing a binary search over an implicit set of candidate ‘critical’ values that contains .
We illustrate the versatility of our general technique by applying it to a variety of g
{"title":"On reverse shortest paths in geometric proximity graphs","authors":"Pankaj K. Agarwal , Matthew J. Katz , Micha Sharir","doi":"10.1016/j.comgeo.2023.102053","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102053","url":null,"abstract":"<div><p>Let <em>S</em> be a set of <em>n</em><span> geometric objects of constant complexity (e.g., points, line segments, disks, ellipses) in </span><span><math><msup><mrow><mi>R</mi></mrow><mrow><mn>2</mn></mrow></msup></math></span>, and let <span><math><mi>ϱ</mi><mo>:</mo><mi>S</mi><mo>×</mo><mi>S</mi><mo>→</mo><msub><mrow><mi>R</mi></mrow><mrow><mo>≥</mo><mn>0</mn></mrow></msub></math></span> be a <em>distance function</em> on <em>S</em>. For a parameter <span><math><mi>r</mi><mo>≥</mo><mn>0</mn></math></span>, we define the <em>proximity graph</em> <span><math><mi>G</mi><mo>(</mo><mi>r</mi><mo>)</mo><mo>=</mo><mo>(</mo><mi>S</mi><mo>,</mo><mi>E</mi><mo>)</mo></math></span> where <span><math><mi>E</mi><mo>=</mo><mo>{</mo><mo>(</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>,</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>)</mo><mo>∈</mo><mi>S</mi><mo>×</mo><mi>S</mi><mo>|</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>≠</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>,</mo><mspace></mspace><mi>ϱ</mi><mo>(</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>,</mo><msub><mrow><mi>e</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>)</mo><mo>≤</mo><mi>r</mi><mo>}</mo></math></span>. Given <em>S</em>, <span><math><mi>s</mi><mo>,</mo><mi>t</mi><mo>∈</mo><mi>S</mi></math></span>, and an integer <span><math><mi>k</mi><mo>≥</mo><mn>1</mn></math></span>, the <em>reverse-shortest-path</em> (RSP) problem asks for computing the smallest value <span><math><msup><mrow><mi>r</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mo>≥</mo><mn>0</mn></math></span> such that <span><math><mi>G</mi><mo>(</mo><msup><mrow><mi>r</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mo>)</mo></math></span> contains a path from <em>s</em> to <em>t</em> of length at most <em>k</em>.</p><p>In this paper we present a general randomized technique that solves the RSP problem efficiently for a large family of geometric objects and distance functions. Using standard, and sometimes more involved, semi-algebraic range-searching techniques, we first give an efficient algorithm for the decision problem, namely, given a value <span><math><mi>r</mi><mo>≥</mo><mn>0</mn></math></span>, determine whether <span><math><mi>G</mi><mo>(</mo><mi>r</mi><mo>)</mo></math></span> contains a path from <em>s</em> to <em>t</em> of length at most <em>k</em>. Next, we adapt our decision algorithm and combine it with a random-sampling method to compute <span><math><msup><mrow><mi>r</mi></mrow><mrow><mo>⁎</mo></mrow></msup></math></span>, by efficiently performing a binary search over an implicit set of <span><math><mi>O</mi><mo>(</mo><msup><mrow><mi>n</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>)</mo></math></span> candidate ‘critical’ values that contains <span><math><msup><mrow><mi>r</mi></mrow><mrow><mo>⁎</mo></mrow></msup></math></span>.</p><p>We illustrate the versatility of our general technique by applying it to a variety of g","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49799333","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-21DOI: 10.1016/j.comgeo.2023.102052
Emily Fox , Hongyao Huang , Benjamin Raichel
In this paper we introduce and formally study the problem of k-clustering with faulty centers. Specifically, we study the faulty versions of k-center, k-median, and k-means clustering, where centers have some probability of not existing, as opposed to prior work where clients had some probability of not existing. For all three problems we provide fixed parameter tractable algorithms, in the parameters k, d, and ε, that -approximate the minimum expected cost solutions for points in d dimensional Euclidean space. For Faulty k-center we additionally provide a 5-approximation for general metrics. Significantly, all of our algorithms have only a linear dependence on n.
{"title":"Clustering with faulty centers","authors":"Emily Fox , Hongyao Huang , Benjamin Raichel","doi":"10.1016/j.comgeo.2023.102052","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102052","url":null,"abstract":"<div><p>In this paper we introduce and formally study the problem of <em>k</em>-clustering with faulty centers. Specifically, we study the faulty versions of <em>k</em>-center, <em>k</em>-median, and <em>k</em><span>-means clustering, where centers have some probability<span> of not existing, as opposed to prior work where clients had some probability of not existing. For all three problems we provide fixed parameter tractable algorithms, in the parameters </span></span><em>k</em>, <em>d</em>, and <em>ε</em>, that <span><math><mo>(</mo><mn>1</mn><mo>+</mo><mi>ε</mi><mo>)</mo></math></span>-approximate the minimum expected cost solutions for points in <em>d</em><span> dimensional Euclidean space. For Faulty </span><em>k</em><span>-center we additionally provide a 5-approximation for general metrics. Significantly, all of our algorithms have only a linear dependence on </span><em>n</em>.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49841451","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-16DOI: 10.1016/j.comgeo.2023.102051
Abolfazl Poureidi , Mohammad Farshi
Let be a real number. A geometric t-spanner is a geometric graph for a point set in with straight line segments between vertices such that the ratio of the shortest-path distance between every pair of vertices in the graph (with Euclidean edge lengths) to their actual Euclidean distance is at most t.
An imprecise point set is modeled by a set R of regions in . If one chooses a point inside each region of R, then the resulting point set is called a precise instance from R. An imprecise t-spanner for an imprecise point set R is a graph such that for each precise instance S from R, graph , where is the set of edges corresponding to E and S, is a t-spanner.
In this paper, we show an imprecise point set R of n straight-line segments in the plane such that any imprecise t-spanner for R has edges. Then, we give an algorithm that computes an imprecise t-spanner for a set of n pairwise disjoint d-dimensional balls with arbitrary sizes. This imprecise t-spanner has edges and can be computed in time. Finally, we show that given an imprecise spanner, finding a precise instance such that its corresponding precise spanner has minimum dilation between all possible precise instances of the imprecise spanner is NP-hard, no matter if crossing edges are allowed or not.
{"title":"On algorithmic complexity of imprecise spanners","authors":"Abolfazl Poureidi , Mohammad Farshi","doi":"10.1016/j.comgeo.2023.102051","DOIUrl":"10.1016/j.comgeo.2023.102051","url":null,"abstract":"<div><p>Let <span><math><mi>t</mi><mo>></mo><mn>1</mn></math></span> be a real number. A geometric <em>t</em><span>-spanner is a geometric graph for a point set in </span><span><math><msup><mrow><mi>R</mi></mrow><mrow><mi>d</mi></mrow></msup></math></span><span> with straight line segments between vertices such that the ratio of the shortest-path distance between every pair of vertices in the graph (with Euclidean edge lengths) to their actual Euclidean distance is at most </span><em>t</em>.</p><p>An imprecise point set is modeled by a set <em>R</em> of regions in <span><math><msup><mrow><mi>R</mi></mrow><mrow><mi>d</mi></mrow></msup></math></span>. If one chooses a point inside each region of <em>R</em>, then the resulting point set is called a precise instance from <em>R</em>. An imprecise <em>t</em>-spanner for an imprecise point set <em>R</em> is a graph <span><math><mi>G</mi><mo>=</mo><mo>(</mo><mi>R</mi><mo>,</mo><mi>E</mi><mo>)</mo></math></span> such that for each precise instance <em>S</em> from <em>R</em>, graph <span><math><msub><mrow><mi>G</mi></mrow><mrow><mi>S</mi></mrow></msub><mo>=</mo><mo>(</mo><mi>S</mi><mo>,</mo><msub><mrow><mi>E</mi></mrow><mrow><mi>S</mi></mrow></msub><mo>)</mo></math></span>, where <span><math><msub><mrow><mi>E</mi></mrow><mrow><mi>S</mi></mrow></msub></math></span> is the set of edges corresponding to <em>E</em> and <em>S</em>, is a <em>t</em>-spanner.</p><p>In this paper, we show an imprecise point set <em>R</em> of <em>n</em> straight-line segments in the plane such that any imprecise <em>t</em>-spanner for <em>R</em> has <span><math><mi>Ω</mi><mo>(</mo><msup><mrow><mi>n</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>)</mo></math></span> edges. Then, we give an algorithm that computes an imprecise <em>t</em>-spanner for a set of <em>n</em><span> pairwise disjoint </span><em>d</em>-dimensional balls with arbitrary sizes. This imprecise <em>t</em>-spanner has <span><math><mi>O</mi><mo>(</mo><mi>n</mi><mo>/</mo><msup><mrow><mo>(</mo><mi>t</mi><mo>−</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>d</mi></mrow></msup><mo>)</mo></math></span> edges and can be computed in <span><math><mi>O</mi><mo>(</mo><mi>n</mi><mi>log</mi><mo></mo><mi>n</mi><mo>/</mo><msup><mrow><mo>(</mo><mi>t</mi><mo>−</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>d</mi></mrow></msup><mo>)</mo></math></span> time. Finally, we show that given an imprecise spanner, finding a precise instance such that its corresponding precise spanner has minimum dilation between all possible precise instances of the imprecise spanner is NP-hard, no matter if crossing edges are allowed or not.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47292577","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-01DOI: 10.1016/j.comgeo.2023.101996
Joachim Gudmundsson , Yuan Sha
We study the problem of augmenting a metric graph by adding k edges while minimizing the radius of the augmented graph. We give a simple 3-approximation algorithm and show that there is no polynomial-time -approximation algorithm, for any , unless .
We also give two exact algorithms for the special case when the input graph is a tree, one of which is generalized to handle metric graphs with bounded treewidth.
{"title":"Augmenting graphs to minimize the radius","authors":"Joachim Gudmundsson , Yuan Sha","doi":"10.1016/j.comgeo.2023.101996","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.101996","url":null,"abstract":"<div><p>We study the problem of augmenting a metric graph by adding <em>k</em> edges while minimizing the radius of the augmented graph. We give a simple 3-approximation algorithm and show that there is no polynomial-time <span><math><mo>(</mo><mn>5</mn><mo>/</mo><mn>3</mn><mo>−</mo><mi>ϵ</mi><mo>)</mo></math></span>-approximation algorithm, for any <span><math><mi>ϵ</mi><mo>></mo><mn>0</mn></math></span>, unless <span><math><mi>P</mi><mo>=</mo><mi>N</mi><mi>P</mi></math></span>.</p><p>We also give two exact algorithms for the special case when the input graph is a tree, one of which is generalized to handle metric graphs with bounded treewidth.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49845789","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper is part of an ongoing endeavor to bring the theory of fair division closer to practice by handling requirements from real-life applications. We focus on two requirements originating from the division of land estates: (1) each agent should receive a plot of a usable geometric shape, and (2) plots of different agents must be physically separated. With these requirements, the classic fairness notion of proportionality is impractical, since it may be impossible to attain any multiplicative approximation of it. In contrast, the ordinal maximin share approximation, introduced by Budish in 2011, provides meaningful fairness guarantees. We prove upper and lower bounds on achievable maximin share guarantees when the usable shapes are squares, fat rectangles, or arbitrary axis-aligned rectangles, and explore the algorithmic and query complexity of finding fair partitions in this setting. Our work makes use of tools and concepts from computational geometry such as independent sets of rectangles and guillotine partitions.
{"title":"Keep your distance: Land division with separation","authors":"Edith Elkind , Erel Segal-Halevi , Warut Suksompong","doi":"10.1016/j.comgeo.2023.102006","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102006","url":null,"abstract":"<div><p>This paper is part of an ongoing endeavor to bring the theory of fair division closer to practice by handling requirements from real-life applications. We focus on two requirements originating from the division of land estates: (1) each agent should receive a plot of a usable geometric shape, and (2) plots of different agents must be physically separated. With these requirements, the classic fairness notion of <em>proportionality</em> is impractical, since it may be impossible to attain any multiplicative approximation of it. In contrast, the <em>ordinal maximin share approximation</em>, introduced by Budish in 2011, provides meaningful fairness guarantees. We prove upper and lower bounds on achievable maximin share guarantees when the usable shapes are squares, fat rectangles, or arbitrary axis-aligned rectangles, and explore the algorithmic and query complexity of finding fair partitions in this setting. Our work makes use of tools and concepts from computational geometry such as independent sets of rectangles and guillotine partitions.</p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49845790","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-01DOI: 10.1016/j.comgeo.2023.101995
Erik D. Demaine, Martin L. Demaine, Yevhenii Diomidov, Tonan Kamata, Ryuhei Uehara, Hanyu Alice Zhang
We show that several classes of polyhedra are joined by a sequence of refolding steps, where each refolding step unfolds the current polyhedron (allowing cuts anywhere on the surface and allowing overlap) and folds that unfolding into exactly the next polyhedron; in other words, a polyhedron is refoldable into another polyhedron if they share a common unfolding. Specifically, assuming equal surface area, we prove that (1) any two tetramonohedra are refoldable to each other, (2) any doubly covered triangle is refoldable to a tetramonohedron, (3) any (augmented) regular prismatoid and doubly covered regular polygon is refoldable to a tetramonohedron, (4) any tetrahedron has a 3-step refolding sequence to a tetramonohedron, and (5) the regular dodecahedron has a 4-step refolding sequence to a tetramonohedron. In particular, we obtain a ≤6-step refolding sequence between any pair of Platonic solids, applying (5) for the dodecahedron and (1) and/or (2) for all other Platonic solids. As far as the authors know, this is the first result about common unfolding involving the regular dodecahedron.
{"title":"Any platonic solid can transform to another by O(1) refoldings","authors":"Erik D. Demaine, Martin L. Demaine, Yevhenii Diomidov, Tonan Kamata, Ryuhei Uehara, Hanyu Alice Zhang","doi":"10.1016/j.comgeo.2023.101995","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.101995","url":null,"abstract":"<div><p><span>We show that several classes of polyhedra are joined by a sequence of </span><span><math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math></span><span> refolding steps, where each refolding step unfolds the current polyhedron (allowing cuts anywhere on the surface and allowing overlap) and folds that unfolding into exactly the next polyhedron; in other words, a polyhedron is refoldable into another polyhedron if they share a common unfolding. Specifically, assuming equal surface area, we prove that (1) any two tetramonohedra are refoldable to each other, (2) any doubly covered triangle is refoldable to a tetramonohedron, (3) any (augmented) regular prismatoid and doubly covered regular polygon<span> is refoldable to a tetramonohedron, (4) any tetrahedron<span> has a 3-step refolding sequence to a tetramonohedron, and (5) the regular dodecahedron<span> has a 4-step refolding sequence to a tetramonohedron. In particular, we obtain a ≤6-step refolding sequence between any pair of Platonic solids, applying (5) for the dodecahedron and (1) and/or (2) for all other Platonic solids. As far as the authors know, this is the first result about common unfolding involving the regular dodecahedron.</span></span></span></span></p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49845788","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-01DOI: 10.1016/j.comgeo.2023.102007
Minati De , Abhiruk Lahiri
In this paper, we study two classic optimization problems: minimum geometric dominating set and set cover. In the dominating-set problem, for a given set of objects in the plane as input, the objective is to choose a minimum number of input objects such that every input object is dominated by the chosen set of objects. Here, we say that one object is dominated by another if their intersection is nonempty. For the second problem, for a given set of points and objects in the plane, the objective is to choose a minimum number of objects to cover all the points. This is a particular version of the set-cover problem.
Both problems have been well-studied, subject to various restrictions on the input objects. These problems are -hard for object sets consisting of axis-parallel rectangles, ellipses, α-fat objects of constant description complexity, and convex polygons. On the other hand, s (polynomial time approximation schemes) are known for object sets consisting of disks or unit squares. Surprisingly, a was unknown even for arbitrary squares. For both problems obtaining a remains open for a large class of objects.
For the dominating-set problem, we prove that a popular local-search algorithm leads to a approximation for a family of homothets of a convex object (which includes arbitrary squares, k-regular polygons, translated and scaled copies of a convex set, etc.) in time. On the other hand, the same approach leads to a for the geometric covering problem when the objects are convex pseudodisks (which include disks, unit height rectangles, homothetic convex objects, etc.). Consequently, we obtain an easy-to-implement approximation algorithm for both problems for a large class of objects, significantly improving the best-known approximation guarantees.
{"title":"Geometric dominating-set and set-cover via local-search","authors":"Minati De , Abhiruk Lahiri","doi":"10.1016/j.comgeo.2023.102007","DOIUrl":"https://doi.org/10.1016/j.comgeo.2023.102007","url":null,"abstract":"<div><p>In this paper, we study two classic optimization problems<span>: minimum geometric dominating set and set cover. In the dominating-set problem, for a given set of objects in the plane as input, the objective is to choose a minimum number of input objects such that every input object is dominated by the chosen set of objects. Here, we say that one object is dominated by another if their intersection is nonempty. For the second problem, for a given set of points and objects in the plane, the objective is to choose a minimum number of objects to cover all the points. This is a particular version of the set-cover problem.</span></p><p>Both problems have been well-studied, subject to various restrictions on the input objects. These problems are <span><math><mi>APX</mi></math></span><span>-hard for object sets consisting of axis-parallel rectangles, ellipses, </span><em>α</em><span>-fat objects of constant description complexity, and convex polygons. On the other hand, </span><span><math><mi>PTAS</mi></math></span><span>s (polynomial time approximation schemes) are known for object sets consisting of disks or unit squares. Surprisingly, a </span><span><math><mi>PTAS</mi></math></span> was unknown even for arbitrary squares. For both problems obtaining a <span><math><mi>PTAS</mi></math></span> remains open for a large class of objects.</p><p>For the dominating-set problem, we prove that a popular local-search algorithm leads to a <span><math><mo>(</mo><mn>1</mn><mo>+</mo><mi>ε</mi><mo>)</mo></math></span> approximation for a family of homothets of a convex object (which includes arbitrary squares, <em>k</em><span>-regular polygons, translated and scaled copies of a convex set, etc.) in </span><span><math><msup><mrow><mi>n</mi></mrow><mrow><mi>O</mi><mo>(</mo><mn>1</mn><mo>/</mo><msup><mrow><mi>ε</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>)</mo></mrow></msup></math></span> time. On the other hand, the same approach leads to a <span><math><mi>PTAS</mi></math></span><span> for the geometric covering problem<span> when the objects are convex pseudodisks (which include disks, unit height rectangles, homothetic convex objects, etc.). Consequently, we obtain an easy-to-implement approximation algorithm for both problems for a large class of objects, significantly improving the best-known approximation guarantees.</span></span></p></div>","PeriodicalId":51001,"journal":{"name":"Computational Geometry-Theory and Applications","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49882871","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"计算机科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}