6 research outputs found
The Zigzag Path of a Pseudo-Triangulation
We define the zigzag path of a pseudo-triangulation, a concept generalizing the path of a triangulation of a point set. The pseudotriangulation zigzag path allows us to use divide-and-conquer type of approaches for suitable (i.e., decomposable) problems on pseudo-triangulations. For this we provide an algorithm that enumerates all pseudotriangulation zigzag paths (of all pseudo-triangulations of a given point set with respect to a given line) in O(n2) time per path and O(n2) space, where n is the number of points. We illustrate applications of our scheme which include a novel algorithm to count the number of pseudotriangulations of a point set. © Springer-Verlag Berlin Heidelberg 2003
Counting Triangulations and other Crossing-Free Structures Approximately
We consider the problem of counting straight-edge triangulations of a given
set of points in the plane. Until very recently it was not known
whether the exact number of triangulations of can be computed
asymptotically faster than by enumerating all triangulations. We now know that
the number of triangulations of can be computed in time,
which is less than the lower bound of on the number of
triangulations of any point set. In this paper we address the question of
whether one can approximately count triangulations in sub-exponential time. We
present an algorithm with sub-exponential running time and sub-exponential
approximation ratio, that is, denoting by the output of our
algorithm, and by the exact number of triangulations of , for some
positive constant , we prove that . This is the first algorithm that in sub-exponential time computes a
-approximation of the base of the number of triangulations, more
precisely, . Our algorithm can be
adapted to approximately count other crossing-free structures on , keeping
the quality of approximation and running time intact. In this paper we show how
to do this for matchings and spanning trees.Comment: 19 pages, 2 figures. A preliminary version appeared at CCCG 201
Pseudo-Triangulations, Rigidity and Motion Planning
This paper proposes a combinatorial approach to planning non-colliding trajectories for a polygonal bar-and-joint framework with n vertices. It is based on a new class of simple motions induced by expansive one-degree-of-freedom mechanisms, which guarantee noncollisions by moving all points away from each other. Their combinatorial structure is captured by pointed pseudo-triangulations, a class of embedded planar graphs for which we give several equivalent characterizations and exhibit rich rigidity theoretic properties. The main application is an efficient algorithm for the Carpenter\u27s Rule Problem: convexify a simple bar-and-joint planar polygonal linkage using only non-self-intersecting planar motions. A step of the algorithm consists in moving a pseudo-triangulation-based mechanism along its unique trajectory in configuration space until two adjacent edges align. At the alignment event, a local alteration restores the pseudo-triangulation. The motion continues for O(n3) steps until all the points are in convex position. © 2005 Springer Science+Business Media, Inc
Selected topics in algorithmic geometry
Let P be a set of n points on the plane with no three points on a line. A crossing-free structure on P is a straight-edge plane graph whose vertex set is P. In this thesis we consider problems of two different topics in the area of algorithmic geometry: Geometry using Steiner points, and counting algorithms. These topics have certain crossing-free structures on P as our primary objects of study. Our results can roughly be described as follows:
i) Given a k-coloring of P, with k >= 3 colors, we will show how to construct a set of Steiner points S = S(P) such that a k-colored quadrangulation can always be constructed on (P U S). The bound we show of |S| significantly improves on previously known results.
ii) We also show how to construct a se S = S(P) of Steiner points such that a triangulation of (P U S) having all its vertices of even (odd) degree can always be constructed. We show that |S| <= n/3 + c, where c is a constant. We also look at other variants of this problem.
iii) With respect to counting algorithms, we show new algorithms for counting triangulations, pseudo-triangulations, crossing-free matchings and crossing-free spanning cycles on P. Our algorithms are simple and allow good analysis of their running times. These algorithms significantly improve over previously known results. We also show an algorithm that counts triangulations approximately, and a hardness result of a particular instance of the problem of counting triangulations exactly.
iv) We show experiments comparing our algorithms for counting triangulations with another well-known algorithm that is supposed to be very fast in practice.Sei P eine Menge von n Punkte in der Ebene, so dass keine drei Punkten auf einer Geraden liegen. Eine kreuzungsfreie Struktur von P ist ein geradliniger ebener Graph, der P als Knotenmenge hat. In dieser Dissertation behandeln wir zwei verschiedene Problemkreise auf dem Gebiet der algorithmischen Geometrie: Geometrie mit Steinerpunkten und Anzahl bestimmende Algorithmen auf P und auf gewissen kreuzungsfreien Strukturen von P. Unsere Resultate können wie folgt beschrieben werden:
i) Gegeben sei eine k-FĂ€rbung von P, mit k >= 3 Farben. Es wird gezeigt, wie eine Menge S = S(P) von Steiner Punkten konstruiert werden kann, die die Konstruktion einer k-gefĂ€rbten Quadrangulierung von (P U S) ermöglicht. Die von uns gezeigte Schranke fĂŒr |S| verbessert die bisher bekannte Schranke.
ii) Gezeigt wird auch die Konstruktion einer Menge S = S(P) von Steiner Punkten, so dass eine Triangulierung von (P U S) konstruiert werden kann, bei der der Grad aller Knoten gerade (ungerade) ist. Wir zeigen, dass |S| <= n/3 + c möglich ist, wobei c eine Konstante ist. Wir betrachten auch andere Varianten dieses Problems.
iii) Was die Anzahl bestimmenden Algorithmen betrifft, zeigen wir neue Algorithmen, um Triangulierungen, Pseudotriangulierungen, kreuzungsfreie Matchings und kreuzungsfreie aufspannende Zyklen von P zu zÀhlen. Unsere Algorithmen sind einfach und lassen eine gute Analyse der Laufzeiten zu. Diese neuen Algorithmen verbessern wesentlich die bisherigen Ergebnisse. Weiter zeigen wir einen Algorithmus, der Triangulierungen approximativ zÀhlt, und bestimmen die KomplexitÀtsklasse einer bestimmten Variante des Problems des exakten ZÀhlens von Triangulierungen.
iv) Wir zeigen Experimente, die unsere triangulierungszÀhlenden Algorithmen mit einem anderen bekannten Algorithmus vergleichen, der in der Praxis als besonders schnell bekannt ist
The zigzag path of a pseudo-triangulation
We define the zigzag path of a pseudo-triangulation, a concept generalizing the path of a triangulation of a point set. The pseudo-triangulation zigzag path allows us to use divide-and-conquer type of approaches for suitable (i.e., decomposable) problems on pseudo-triangulations. For this we provide an algorithm that enumerates all pseudo-triangulation zigzag paths (of all pseudo-triangulations of a given point set with respect to a given line) in O(n 2) time per path and O(n 2) space, where n is the number of points. We illustrate applications of our scheme which include a novel algorithm to count the number of pseudo-triangulations of a point set
The Zigzag Path of a Pseudo-Triangulation
We define the zigzag path of a pseudo-triangulation, a concept generalizing the path of a triangulation of a point set. The pseudo-triangulation zigzag path allows us to use divide-and-conquer type of approaches for suitable (i.e., decomposable) problems on pseudo-triangulations