542 research outputs found
Geometric Permutations of Non-Overlapping Unit Balls Revisited
Given four congruent balls in that have disjoint
interior and admit a line that intersects them in the order , we show
that the distance between the centers of consecutive balls is smaller than the
distance between the centers of and . This allows us to give a new short
proof that interior-disjoint congruent balls admit at most three geometric
permutations, two if . We also make a conjecture that would imply that
such balls admit at most two geometric permutations, and show that if
the conjecture is false, then there is a counter-example of a highly degenerate
nature
Determinantal Processes and Independence
We give a probabilistic introduction to determinantal and permanental point
processes. Determinantal processes arise in physics (fermions, eigenvalues of
random matrices) and in combinatorics (nonintersecting paths, random spanning
trees). They have the striking property that the number of points in a region
is a sum of independent Bernoulli random variables, with parameters which
are eigenvalues of the relevant operator on . Moreover, any
determinantal process can be represented as a mixture of determinantal
projection processes. We give a simple explanation for these known facts, and
establish analogous representations for permanental processes, with geometric
variables replacing the Bernoulli variables. These representations lead to
simple proofs of existence criteria and central limit theorems, and unify known
results on the distribution of absolute values in certain processes with
radially symmetric distributions.Comment: Published at http://dx.doi.org/10.1214/154957806000000078 in the
Probability Surveys (http://www.i-journals.org/ps/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Sparse Modeling for Image and Vision Processing
In recent years, a large amount of multi-disciplinary research has been
conducted on sparse models and their applications. In statistics and machine
learning, the sparsity principle is used to perform model selection---that is,
automatically selecting a simple model among a large collection of them. In
signal processing, sparse coding consists of representing data with linear
combinations of a few dictionary elements. Subsequently, the corresponding
tools have been widely adopted by several scientific communities such as
neuroscience, bioinformatics, or computer vision. The goal of this monograph is
to offer a self-contained view of sparse modeling for visual recognition and
image processing. More specifically, we focus on applications where the
dictionary is learned and adapted to data, yielding a compact representation
that has been successful in various contexts.Comment: 205 pages, to appear in Foundations and Trends in Computer Graphics
and Visio
Twin-width I: tractable FO model checking
Inspired by a width invariant defined on permutations by Guillemot and Marx
[SODA '14], we introduce the notion of twin-width on graphs and on matrices.
Proper minor-closed classes, bounded rank-width graphs, map graphs, -free
unit -dimensional ball graphs, posets with antichains of bounded size, and
proper subclasses of dimension-2 posets all have bounded twin-width. On all
these classes (except map graphs without geometric embedding) we show how to
compute in polynomial time a sequence of -contractions, witness that the
twin-width is at most . We show that FO model checking, that is deciding if
a given first-order formula evaluates to true for a given binary
structure on a domain , is FPT in on classes of bounded
twin-width, provided the witness is given. More precisely, being given a
-contraction sequence for , our algorithm runs in time where is a computable but non-elementary function. We also prove that
bounded twin-width is preserved by FO interpretations and transductions
(allowing operations such as squaring or complementing a graph). This unifies
and significantly extends the knowledge on fixed-parameter tractability of FO
model checking on non-monotone classes, such as the FPT algorithm on
bounded-width posets by Gajarsk\'y et al. [FOCS '15].Comment: 49 pages, 9 figure
Statistical Mechanics of the Uniform Electron Gas
In this paper we define and study the classical Uniform Electron Gas (UEG), a
system of infinitely many electrons whose density is constant everywhere in
space. The UEG is defined differently from Jellium, which has a positive
constant background but no constraint on the density. We prove that the UEG
arises in Density Functional Theory in the limit of a slowly varying density,
minimizing the indirect Coulomb energy. We also construct the quantum UEG and
compare it to the classical UEG at low density.Comment: Final version to appear in J. Ec. polytech. Mat
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