10,910 research outputs found
Regularity of the Einstein Equations at Future Null Infinity
When Einstein's equations for an asymptotically flat, vacuum spacetime are
reexpressed in terms of an appropriate conformal metric that is regular at
(future) null infinity, they develop apparently singular terms in the
associated conformal factor and thus appear to be ill-behaved at this
(exterior) boundary. In this article however we show, through an enforcement of
the Hamiltonian and momentum constraints to the needed order in a Taylor
expansion, that these apparently singular terms are not only regular at the
boundary but can in fact be explicitly evaluated there in terms of conformally
regular geometric data. Though we employ a rather rigidly constrained and gauge
fixed formulation of the field equations, we discuss the extent to which we
expect our results to have a more 'universal' significance and, in particular,
to be applicable, after minor modifications, to alternative formulations.Comment: 43 pages, no figures, AMS-TeX. Minor revisions, updated to agree with
published versio
Regularity Preserving but not Reflecting Encodings
Encodings, that is, injective functions from words to words, have been
studied extensively in several settings. In computability theory the notion of
encoding is crucial for defining computability on arbitrary domains, as well as
for comparing the power of models of computation. In language theory much
attention has been devoted to regularity preserving functions.
A natural question arising in these contexts is: Is there a bijective
encoding such that its image function preserves regularity of languages, but
its pre-image function does not? Our main result answers this question in the
affirmative: For every countable class C of languages there exists a bijective
encoding f such that for every language L in C its image f[L] is regular.
Our construction of such encodings has several noteworthy consequences.
Firstly, anomalies arise when models of computation are compared with respect
to a known concept of implementation that is based on encodings which are not
required to be computable: Every countable decision model can be implemented,
in this sense, by finite-state automata, even via bijective encodings. Hence
deterministic finite-state automata would be equally powerful as Turing machine
deciders.
A second consequence concerns the recognizability of sets of natural numbers
via number representations and finite automata. A set of numbers is said to be
recognizable with respect to a representation if an automaton accepts the
language of representations. Our result entails that there is one number
representation with respect to which every recursive set is recognizable
Regular cell complexes in total positivity
This paper proves a conjecture of Fomin and Shapiro that their combinatorial
model for any Bruhat interval is a regular CW complex which is homeomorphic to
a ball. The model consists of a stratified space which may be regarded as the
link of an open cell intersected with a larger closed cell, all within the
totally nonnegative part of the unipotent radical of an algebraic group. A
parametrization due to Lusztig turns out to have all the requisite features to
provide the attaching maps. A key ingredient is a new, readily verifiable
criterion for which finite CW complexes are regular involving an interplay of
topology with combinatorics.Comment: accepted to Inventiones Mathematicae; 60 pages; substantially revised
from earlier version
Natural hp-BEM for the electric field integral equation with singular solutions
We apply the hp-version of the boundary element method (BEM) for the
numerical solution of the electric field integral equation (EFIE) on a
Lipschitz polyhedral surface G. The underlying meshes are supposed to be
quasi-uniform triangulations of G, and the approximations are based on either
Raviart-Thomas or Brezzi-Douglas-Marini families of surface elements.
Non-smoothness of G leads to singularities in the solution of the EFIE,
severely affecting convergence rates of the BEM. However, the singular
behaviour of the solution can be explicitly specified using a finite set of
power functions (vertex-, edge-, and vertex-edge singularities). In this paper
we use this fact to perform an a priori error analysis of the hp-BEM on
quasi-uniform meshes. We prove precise error estimates in terms of the
polynomial degree p, the mesh size h, and the singularity exponents.Comment: 17 page
Projected likelihood contrasts for testing homogeneity in finite mixture models with nuisance parameters
This paper develops a test for homogeneity in finite mixture models where the
mixing proportions are known a priori (taken to be 0.5) and a common nuisance
parameter is present. Statistical tests based on the notion of Projected
Likelihood Contrasts (PLC) are considered. The PLC is a slight modification of
the usual likelihood ratio statistic or the Wilk's and is similar in
spirit to the Rao's score test. Theoretical investigations have been carried
out to understand the large sample statistical properties of these tests.
Simulation studies have been carried out to understand the behavior of the null
distribution of the PLC statistic in the case of Gaussian mixtures with unknown
means (common variance as nuisance parameter) and unknown variances (common
mean as nuisance parameter). The results are in conformity with the theoretical
results obtained. Power functions of these tests have been evaluated based on
simulations from Gaussian mixtures.Comment: Published in at http://dx.doi.org/10.1214/193940307000000194 the IMS
Collections (http://www.imstat.org/publications/imscollections.htm) by the
Institute of Mathematical Statistics (http://www.imstat.org
The bigravity black hole and its thermodynamics
We argue that the Isham-Storey exact solution to bigravity does not describe
black holes because the horizon is a singular surface. However, this is not a
generic property of bigravity, but a property of a particular potential. More
general potentials do accept regular black holes. For regular black holes, we
compute the total energy and thermodynamical parameters. Phase transitions
occur for certain critical temperatures. We also find a novel region on phase
space describing up to 4 allowed states for a given temperature.Comment: 14 pages, 9 figure
Indirect Image Registration with Large Diffeomorphic Deformations
The paper adapts the large deformation diffeomorphic metric mapping framework
for image registration to the indirect setting where a template is registered
against a target that is given through indirect noisy observations. The
registration uses diffeomorphisms that transform the template through a (group)
action. These diffeomorphisms are generated by solving a flow equation that is
defined by a velocity field with certain regularity. The theoretical analysis
includes a proof that indirect image registration has solutions (existence)
that are stable and that converge as the data error tends so zero, so it
becomes a well-defined regularization method. The paper concludes with examples
of indirect image registration in 2D tomography with very sparse and/or highly
noisy data.Comment: 43 pages, 4 figures, 1 table; revise
Mutation of Directed Graphs -- Corresponding Regular Expressions and Complexity of Their Generation
Directed graphs (DG), interpreted as state transition diagrams, are
traditionally used to represent finite-state automata (FSA). In the context of
formal languages, both FSA and regular expressions (RE) are equivalent in that
they accept and generate, respectively, type-3 (regular) languages. Based on
our previous work, this paper analyzes effects of graph manipulations on
corresponding RE. In this present, starting stage we assume that the DG under
consideration contains no cycles. Graph manipulation is performed by deleting
or inserting of nodes or arcs. Combined and/or multiple application of these
basic operators enable a great variety of transformations of DG (and
corresponding RE) that can be seen as mutants of the original DG (and
corresponding RE). DG are popular for modeling complex systems; however they
easily become intractable if the system under consideration is complex and/or
large. In such situations, we propose to switch to corresponding RE in order to
benefit from their compact format for modeling and algebraic operations for
analysis. The results of the study are of great potential interest to mutation
testing
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