313 research outputs found
The Darboux-Backlund transformation for the static 2-dimensional continuum Heisenberg chain
We construct the Darboux-Backlund transformation for the sigma model
describing static configurations of the 2-dimensional classical continuum
Heisenberg chain. The transformation is characterized by a non-trivial
normalization matrix depending on the background solution. In order to obtain
the transformation we use a new, more general, spectral problem.Comment: 12 page
On analytic descriptions of two-dimensional surfaces associated with the CP^(N-1) sigma model
We study analytic descriptions of conformal immersions of the Riemann sphere
S^2 into the CP^(N-1) sigma model. In particular, an explicit expression for
two-dimensional (2-D) surfaces, obtained from the generalized Weierstrass
formula, is given. It is also demonstrated that these surfaces coincide with
the ones obtained from the Sym-Tafel formula. These two approaches correspond
to parametrizations of one and the same surface in R^(N^2-1).Comment: 6 page
The geometric sense of R. Sasaki connection
For the Riemannian manifold two special connections on the sum of the
tangent bundle and the trivial one-dimensional bundle are constructed.
These connections are flat if and only if the space has a constant
sectional curvature . The geometric explanation of this property is
given. This construction gives a coordinate free many-dimensional
generalization of the connection from the paper: R. Sasaki 1979 Soliton
equations and pseudospherical surfaces, Nuclear Phys., {\bf 154 B}, pp.
343-357. It is shown that these connections are in close relation with the
imbedding of into Euclidean or pseudoeuclidean -dimension
spaces.Comment: 7 pages, the key reference to the paper of Min-Oo is included in the
second versio
Pseudospherical surfaces on time scales: a geometric definition and the spectral approach
We define and discuss the notion of pseudospherical surfaces in asymptotic
coordinates on time scales. Thus we extend well known notions of discrete
pseudospherical surfaces and smooth pseudosperical surfaces on more exotic
domains (e.g, the Cantor set). In particular, we present a new expression for
the discrete Gaussian curvature which turns out to be valid for asymptotic nets
on any time scale. We show that asymptotic Chebyshev nets on an arbitrary time
scale have constant negative Gaussian curvature. We present also the
quaternion-valued spectral problem (the Lax pair) and the Darboux-Backlund
transformation for pseudospherical surfaces (in asymptotic coordinates) on
arbitrary time scales.Comment: 20 page
Comparing Computational Architectures for Automated Journalism
The majority of NLG systems have been designed following either a
template-based or a pipeline-based architecture. Recent neural models for
data-to-text generation have been proposed with an end-to-end deep learning
flavor, which handles non-linguistic input in natural language without explicit
intermediary representations. This study compares the most often employed
methods for generating Brazilian Portuguese texts from structured data. Results
suggest that explicit intermediate steps in the generation process produce
better texts than the ones generated by neural end-to-end architectures,
avoiding data hallucination while better generalizing to unseen inputs. Code
and corpus are publicly available.Comment: Accepted at the 19th National Meeting on Artificial and Computational
Intelligence (ENIAC 2022
Finite-gap Solutions of the Vortex Filament Equation: Isoperiodic Deformations
We study the topology of quasiperiodic solutions of the vortex filament
equation in a neighborhood of multiply covered circles. We construct these
solutions by means of a sequence of isoperiodic deformations, at each step of
which a real double point is "unpinched" to produce a new pair of branch points
and therefore a solution of higher genus. We prove that every step in this
process corresponds to a cabling operation on the previous curve, and we
provide a labelling scheme that matches the deformation data with the knot type
of the resulting filament.Comment: 33 pages, 5 figures; submitted to Journal of Nonlinear Scienc
The ISO SWS on-line system
The software which is currently being developed for the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) is described. The spectrometer has a wide range of capabilities in the 2-45 micron infrared band. SWS contains two independent gratings, one for the long and one for the short wavelength section of the band. With the gratings a spectral resolution of approximately 1000 to approximately 2500 can be obtained. The instrument also contains two Fabry-Perault's yielding a resolution between approximately 1000 and approximately 20000. Software is currently being developed for the acquisition, calibration, and analysis of SWS data. The software is firstly required to run in a pipeline mode without human interaction, to process data as they are received from the telescope. However, both for testing and calibration of the instrument as well as for evaluation of the planned operating procedures the software should also be suitable for interactive use. Thirdly the same software will be used for long term characterization of the instrument. The software must work properly within the environment designed by the European Space Agency (ESA) for the spacecraft operations. As a result strict constraints are put on I/O devices, throughput etc
Potential medicinal value of some South African seaweeds
Eleven macroalgae were collected from the KwaZulu-Natal coast and nineteen species from the cooler Western Cape coast in March and April 2000. Ethanolic and aqueous extracts were made and tested for biological activity in the Cox-1 anti-inflammatory assay, in a nematode mortality bioassay for anthelminthic activity, an IC50 anticancer assay and a MIC antimicrobial assay. The ethanolic extracts were very active in the Cox-1 anti-inflammatory assay for almost all of the species tested. The aqueous extracts were not active. No anthelminthic mortality was detected in extracts from any of the species tested. Many of the extracts had cytotoxic activity against three cancer cell lines tested, with those from representative species of the Chlorophyta and Rhodophyta being the most effective. The extracts had much lower cytotoxic activity when tested on normal mouse fibroblasts (NIH3T3). Extracts from only a few species had antimicrobial activity with those of the Chlorophyta tested being the most effective against both the Gram-positive and Gram-negative bacteria
Scalar second order evolution equations possessing an irreducible sl-valued zero curvature representation
We find all scalar second order evolution equations possessing an
sl-valued zero curvature representation that is not reducible to a proper
subalgebra of sl. None of these zero-curvature representations admits a
parameter.Comment: 10 pages, requires nath.st
Generalized isothermic lattices
We study multidimensional quadrilateral lattices satisfying simultaneously
two integrable constraints: a quadratic constraint and the projective Moutard
constraint. When the lattice is two dimensional and the quadric under
consideration is the Moebius sphere one obtains, after the stereographic
projection, the discrete isothermic surfaces defined by Bobenko and Pinkall by
an algebraic constraint imposed on the (complex) cross-ratio of the circular
lattice. We derive the analogous condition for our generalized isthermic
lattices using Steiner's projective structure of conics and we present basic
geometric constructions which encode integrability of the lattice. In
particular, we introduce the Darboux transformation of the generalized
isothermic lattice and we derive the corresponding Bianchi permutability
principle. Finally, we study two dimensional generalized isothermic lattices,
in particular geometry of their initial boundary value problem.Comment: 19 pages, 11 figures; v2. some typos corrected; v3. new references
added, higlighted similarities and differences with recent papers on the
subjec
- …