18,329 research outputs found
Knotted Defects in Nematic Liquid Crystals
We show that the number of distinct topological states associated to a given
knotted defect, , in a nematic liquid crystal is equal to the determinant of
the link . We give an interpretation of these states, demonstrate how they
may be identified in experiments and describe the consequences for material
behaviour and interactions between multiple knots. We show that stable knots
can be created in a bulk cholesteric and illustrate the topology by classifying
a simulated Hopf link. In addition we give a topological heuristic for the
resolution of strand crossings in defect coarsening processes which allows us
to distinguish topological classes of a given link and to make predictions
about defect crossings in nematic liquid crystals.Comment: 10 pages, 4 figure
Within-guild dietary discrimination from 3-D textural analysis of tooth microwear in insectivorous mammals
Resource exploitation and competition for food are important selective pressures in animal evolution. A number of recent investigations have focused on linkages between diversification, trophic morphology and diet in bats, partly because their roosting habits mean that for many bat species diet can be quantified relatively easily through faecal analysis. Dietary analysis in mammals is otherwise invasive, complicated, time consuming and expensive. Here we present evidence from insectivorous bats that analysis of three-dimensional (3-D) textures of tooth microwear using International Organization for Standardization (ISO) roughness parameters derived from sub-micron surface data provides an additional, powerful tool for investigation of trophic resource exploitation in mammals. Our approach, like scale-sensitive fractal analysis, offers considerable advantages over twodimensional (2-D) methods of microwear analysis, including improvements in robustness, repeatability and comparability of studies. Our results constitute the first analysis of microwear textures in carnivorous mammals based on ISO roughness parameters. They demonstrate that the method is capable of dietary discrimination, even between cryptic species with subtly different diets within trophic guilds, and even when sample sizes are small. We find significant differences in microwear textures between insectivore species whose diet contains different proportions of âhardâ prey (such as beetles) and âsoftâ prey (such as moths), and multivariate analyses are able to distinguish between species with different diets based solely on their tooth microwear textures. Our results show that, compared with previous 2-D analyses of microwear in bats, ISO roughness parameters provide a much more sophisticated characterization of the nature of microwear surfaces and can yield more robust and subtle dietary discrimination. ISO-based textural analysis of tooth microwear thus has a useful role to play, complementing existing approaches, in trophic analysis of mammals, both extant and extinct
Cosmology from Topological Defects
The potential role of cosmic topological defects has raised interest in the
astrophysical community for many years now. In this set of notes, we give an
introduction to the subject of cosmic topological defects and some of their
possible observable signatures. We begin with a review of the basics of general
defect formation and evolution, we briefly comment on some general features of
conducting cosmic strings and vorton formation, as well as on the possible role
of defects as dark energy, to end up with cosmic structure formation from
defects and some specific imprints in the cosmic microwave background radiation
from simulated cosmic strings. A detailed, pedagogical explanation of the
mechanism underlying the tiny level of polarization discovered in the cosmic
microwave background by the DASI collaboration (and recently confirmed by WMAP)
is also given, and a first rough comparison with some predictions from defects
is provided.Comment: Lecture Notes delivered at the Xth Brazilian School on Cosmology and
Gravitation, Mangaratiba, Rio de Janeiro, July 29 - August 9, 2002. To appear
in the proceedings (AIP Press), edited by M. Novello and S. Perez Bergliaffa.
Updated source files with high resolution figures available at
http://www.iafe.uba.ar/relatividad/gangui/xescola
Power Spectra in Global Defect Theories of Cosmic Structure Formation
An efficient technique for computing perturbation power spectra in field
ordering theories of cosmic structure formation is introduced, enabling
computations to be carried out with unprecedented precision. Large scale
simulations are used to measure unequal time correlators of the source stress
energy, taking advantage of scaling during matter and radiation domination, and
causality, to make optimal use of the available dynamic range. The correlators
are then re-expressed in terms of a sum of eigenvector products, a
representation which we argue is optimal, enabling the computation of the final
power spectra to be performed at high accuracy. Microwave anisotropy and matter
perturbation power spectra for global strings, monopoles, textures and
non-topological textures are presented and compared with recent observations.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
Hierarchical Bayesian Detection Algorithm for Early-Universe Relics in the Cosmic Microwave Background
A number of theoretically well-motivated additions to the standard
cosmological model predict weak signatures in the form of spatially localized
sources embedded in the cosmic microwave background (CMB) fluctuations. We
present a hierarchical Bayesian statistical formalism and a complete data
analysis pipeline for testing such scenarios. We derive an accurate
approximation to the full posterior probability distribution over the
parameters defining any theory that predicts sources embedded in the CMB, and
perform an extensive set of tests in order to establish its validity. The
approximation is implemented using a modular algorithm, designed to avoid a
posteriori selection effects, which combines a candidate-detection stage with a
full Bayesian model-selection and parameter-estimation analysis. We apply this
pipeline to theories that predict cosmic textures and bubble collisions,
extending previous analyses by using: (1) adaptive-resolution techniques,
allowing us to probe features of arbitrary size, and (2) optimal filters, which
provide the best possible sensitivity for detecting candidate signatures. We
conclude that the WMAP 7-year data do not favor the addition of either cosmic
textures or bubble collisions to the standard cosmological model, and place
robust constraints on the predicted number of such sources. The expected
numbers of bubble collisions and cosmic textures on the CMB sky within our
detection thresholds are constrained to be fewer than 4.0 and 5.2 at 95%
confidence, respectively.Comment: 34 pages, 18 figures. v3: corrected very minor typos to match
published versio
Automatic Model Based Dataset Generation for Fast and Accurate Crop and Weeds Detection
Selective weeding is one of the key challenges in the field of agriculture
robotics. To accomplish this task, a farm robot should be able to accurately
detect plants and to distinguish them between crop and weeds. Most of the
promising state-of-the-art approaches make use of appearance-based models
trained on large annotated datasets. Unfortunately, creating large agricultural
datasets with pixel-level annotations is an extremely time consuming task,
actually penalizing the usage of data-driven techniques. In this paper, we face
this problem by proposing a novel and effective approach that aims to
dramatically minimize the human intervention needed to train the detection and
classification algorithms. The idea is to procedurally generate large synthetic
training datasets randomizing the key features of the target environment (i.e.,
crop and weed species, type of soil, light conditions). More specifically, by
tuning these model parameters, and exploiting a few real-world textures, it is
possible to render a large amount of realistic views of an artificial
agricultural scenario with no effort. The generated data can be directly used
to train the model or to supplement real-world images. We validate the proposed
methodology by using as testbed a modern deep learning based image segmentation
architecture. We compare the classification results obtained using both real
and synthetic images as training data. The reported results confirm the
effectiveness and the potentiality of our approach.Comment: To appear in IEEE/RSJ IROS 201
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