667 research outputs found
Generating Kernel Aware Polygons
Problems dealing with the generation of random polygons has important applications for evaluating the performance of algorithms on polygonal domain. We review existing algorithms for generating random polygons. We present an algorithm for generating polygons admitting visibility properties. In particular, we propose an algorithm for generating polygons admitting large size kernels. We also present experimental results on generating such polygons
Simple maps, Hurwitz numbers, and Topological Recursion
We introduce the notion of fully simple maps, which are maps with non
self-intersecting disjoint boundaries. In contrast, maps where such a
restriction is not imposed are called ordinary. We study in detail the
combinatorics of fully simple maps with topology of a disk or a cylinder. We
show that the generating series of simple disks is given by the functional
inversion of the generating series of ordinary disks. We also obtain an elegant
formula for cylinders. These relations reproduce the relation between moments
and free cumulants established by Collins et al. math.OA/0606431, and implement
the symplectic transformation on the spectral curve in
the context of topological recursion. We conjecture that the generating series
of fully simple maps are computed by the topological recursion after exchange
of and . We propose an argument to prove this statement conditionally to
a mild version of symplectic invariance for the -hermitian matrix model,
which is believed to be true but has not been proved yet.
Our argument relies on an (unconditional) matrix model interpretation of
fully simple maps, via the formal hermitian matrix model with external field.
We also deduce a universal relation between generating series of fully simple
maps and of ordinary maps, which involves double monotone Hurwitz numbers. In
particular, (ordinary) maps without internal faces -- which are generated by
the Gaussian Unitary Ensemble -- and with boundary perimeters
are strictly monotone double Hurwitz numbers
with ramifications above and above .
Combining with a recent result of Dubrovin et al. math-ph/1612.02333, this
implies an ELSV-like formula for these Hurwitz numbers.Comment: 66 pages, 7 figure
Probability Theory of Random Polygons from the Quaternionic Viewpoint
We build a new probability measure on closed space and plane polygons. The
key construction is a map, given by Knutson and Hausmann using the Hopf map on
quaternions, from the complex Stiefel manifold of 2-frames in n-space to the
space of closed n-gons in 3-space of total length 2. Our probability measure on
polygon space is defined by pushing forward Haar measure on the Stiefel
manifold by this map. A similar construction yields a probability measure on
plane polygons which comes from a real Stiefel manifold.
The edgelengths of polygons sampled according to our measures obey beta
distributions. This makes our polygon measures different from those usually
studied, which have Gaussian or fixed edgelengths. One advantage of our
measures is that we can explicitly compute expectations and moments for
chordlengths and radii of gyration. Another is that direct sampling according
to our measures is fast (linear in the number of edges) and easy to code.
Some of our methods will be of independent interest in studying other
probability measures on polygon spaces. We define an edge set ensemble (ESE) to
be the set of polygons created by rearranging a given set of n edges. A key
theorem gives a formula for the average over an ESE of the squared lengths of
chords skipping k vertices in terms of k, n, and the edgelengths of the
ensemble. This allows one to easily compute expected values of squared
chordlengths and radii of gyration for any probability measure on polygon space
invariant under rearrangements of edges.Comment: Some small typos fixed, added a calculation for the covariance of
edgelengths, added pseudocode for the random polygon sampling algorithm. To
appear in Communications on Pure and Applied Mathematics (CPAM
Computational Geometry Column 42
A compendium of thirty previously published open problems in computational
geometry is presented.Comment: 7 pages; 72 reference
On the enumeration of permutominoes
Although the exact counting and enumeration of polyominoes remain challenging open problems, several positive results were achieved for special classes of polyominoes. We give an algorithm for direct enumeration of permutominoes by size, or, equivalently, for the enumeration of grid orthogonal polygons. We show how the construction technique allows us to derive a simple characterization of the class of convex permutominoes, which has been extensively investigated. The approach extends to other classes, such as the row convex and the directed convex permutominoes.Fondo Europeo de Desarrollo RegionalFundação para a Ciência e a Tecnologi
Approximation Algorithms for Illuminating 1.5D Terrain
We review important algorithmic results for the coverage of 1.5D terrain by point guards. Finding the minimum number of point guards for covering 1.5D terrain is known to be NP-hard. We propose two approximation algorithms for covering 1.5D terrain by a fewer number of point guards. The first algorithm (Greedy Ranking Algorithm) is based on ranking vertices in term of number of visible edges from them. The second algorithm (Greedy Forward Marching Algorithm) works in greedy manner by scanning the terrain from left to right. Both algorithms are implemented in Python 2.7 programming language
Deconstructing Approximate Offsets
We consider the offset-deconstruction problem: Given a polygonal shape Q with
n vertices, can it be expressed, up to a tolerance \eps in Hausdorff distance,
as the Minkowski sum of another polygonal shape P with a disk of fixed radius?
If it does, we also seek a preferably simple-looking solution P; then, P's
offset constitutes an accurate, vertex-reduced, and smoothened approximation of
Q. We give an O(n log n)-time exact decision algorithm that handles any
polygonal shape, assuming the real-RAM model of computation. A variant of the
algorithm, which we have implemented using CGAL, is based on rational
arithmetic and answers the same deconstruction problem up to an uncertainty
parameter \delta; its running time additionally depends on \delta. If the input
shape is found to be approximable, this algorithm also computes an approximate
solution for the problem. It also allows us to solve parameter-optimization
problems induced by the offset-deconstruction problem. For convex shapes, the
complexity of the exact decision algorithm drops to O(n), which is also the
time required to compute a solution P with at most one more vertex than a
vertex-minimal one.Comment: 18 pages, 11 figures, previous version accepted at SoCG 2011,
submitted to DC
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