173 research outputs found
Exact solutions of closed string theory
We review explicitly known exact solutions with Minkowski signature in
closed bosonic string theory. Classical string solutions with space-time
interpretation are represented by conformal sigma models. Two large
(intersecting) classes of solutions are described by gauged WZW models and
`chiral null models' (models with conserved chiral null current). The latter
class includes plane-wave type backgrounds (admitting a covariantly constant
null Killing vector) and backgrounds with two null Killing vectors (e.g.,
fundamental string solution). chiral null models describe some exact
solutions with electromagnetic fields, for example, extreme electric
black holes, charged fundamental strings and their generalisations. In
addition, there exists a class of conformal models representing axially
symmetric stationary magnetic flux tube backgrounds (including, in particular,
the dilatonic Melvin solution). In contrast to spherically symmetric chiral
null models for which the corresponding conformal field theory is not known
explicitly, the magnetic flux tube models (together with some non-semisimple
WZW models) are among the first examples of solvable unitary conformal string
models with non-trivial curved space-time interpretation. For these
models one is able to express the quantum hamiltonian in terms of free fields
and to find explicitly the physical spectrum and string partition function.Comment: 50 pages, harvma
Solvable model of strings in a time-dependent plane-wave background
We investigate a string model defined by a special plane-wave metric ds^2 =
2dudv - l(u) x^2 du^2 + dx^2 with l(u) = k/u^2 and k=const > 0. This metric is
a Penrose limit of some cosmological, Dp-brane and fundamental string
backgrounds. Remarkably, in Rosen coordinates the metric has a ``null
cosmology'' interpretation with flat spatial sections and scale factor which is
a power of the light-cone time u. We show that: (i) This spacetime is a
Lorentzian homogeneous space. In particular, like Minkowski space, it admits a
boost isometry in u,v. (ii) It is an exact solution of string theory when
supplemented by a u-dependent dilaton such that its exponent (i.e. effective
string coupling) goes to zero at u=infinity and at the singularity u=0,
reducing back-reaction effects. (iii) The classical string equations in this
background become linear in the light-cone gauge and can be solved explicitly
in terms of Bessel's functions; thus the string model can be directly
quantized. This allows one to address the issue of singularity at the
string-theory level. We examine the propagation of first-quantized
point-particle and string modes in this time-dependent background. Using
certain analytic continuation prescription we argue that string propagation
through the singularity can be smooth.Comment: 58 pages, latex. v2: several references to related previous work
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On the Singularity Structure and Stability of Plane Waves
We describe various aspects of plane wave backgrounds. In particular, we make
explicit a simple criterion for singularity by establishing a relation between
Brinkmann metric entries and diffeomorphism-invariant curvature information. We
also address the stability of plane wave backgrounds by analyzing the
fluctuations of generic scalar modes. We focus our attention on cases where
after fixing the light-cone gauge the resulting world sheet fields appear to
have negative "mass terms". We nevertheless argue that these backgrounds may be
stable.Comment: 21 pages, 1 figur
Gauged WZW models and Non-abelian duality
We consider WZW models based on the non-semi-simple algebras that they were
recently constructed as contractions of corresponding algebras for semi-simple
groups. We give the explicit expression for the action of these models, as well
as for a generalization of them, and discuss their general properties.
Furthermore we consider gauged WZW models based on these non-semi-simple
algebras and we show that there are equivalent to non-abelian duality
transformations on WZW actions. We also show that a general non-abelian duality
transformation can be thought of as a limiting case of the non-abelian quotient
theory of the direct product of the original action and the WZW action for the
symmetry gauge group . In this action there is no Lagrange multiplier term
that constrains the gauge field strength to vanish. A particular result is that
the gauged WZW action for the coset is equivalent,
in the limit , to the dualized WZW action for with respect
to the subgroup .Comment: 35 pages, harvmac, THU-94/01 (a few minor changes in subsec. 4.1 are
made, a 3rd App. and a Note are added
Expression and trans-specific polymorphism of self-incompatibility RNases in Coffea (Rubiaceae)
Self-incompatibility (SI) is widespread in the angiosperms, but identifying the biochemical components of SI mechanisms has proven to be difficult in most lineages. Coffea (coffee; Rubiaceae) is a genus of old-world tropical understory trees in which the vast majority of diploid species utilize a mechanism of gametophytic self-incompatibility (GSI). The S-RNase GSI system was one of the first SI mechanisms to be biochemically characterized, and likely represents the ancestral Eudicot condition as evidenced by its functional characterization in both asterid (Solanaceae, Plantaginaceae) and rosid (Rosaceae) lineages. The S-RNase GSI mechanism employs the activity of class III RNase T2 proteins to terminate the growth of "self" pollen tubes. Here, we investigate the mechanism of Coffea GSI and specifically examine the potential for homology to S-RNase GSI by sequencing class III RNase T2 genes in populations of 14 African and Madagascan Coffea species and the closely related self-compatible species Psilanthus ebracteolatus. Phylogenetic analyses of these sequences aligned to a diverse sample of plant RNase T2 genes show that the Coffea genome contains at least three class III RNase T2 genes. Patterns of tissue-specific gene expression identify one of these RNase T2 genes as the putative Coffea S-RNase gene. We show that populations of SI Coffea are remarkably polymorphic for putative S-RNase alleles, and exhibit a persistent pattern of trans-specific polymorphism characteristic of all S-RNase genes previously isolated from GSI Eudicot lineages. We thus conclude that Coffea GSI is most likely homologous to the classic Eudicot S-RNase system, which was retained since the divergence of the Rubiaceae lineage from an ancient SI Eudicot ancestor, nearly 90 million years ago.United States National Science Foundation [0849186]; Society of Systematic Biologists; American Society of Plant Taxonomists; Duke University Graduate Schoolinfo:eu-repo/semantics/publishedVersio
D-branes in λ-deformations
We show that the geometric interpretation of D-branes in WZW models as twisted conjugacy classes persists in the --deformed theory. We obtain such configurations by demanding that a monodromy matrix constructed from the Lax connection of the --deformed theory continues to produce conserved charges in the presence of boundaries. In this way the D-brane configurations obtained correspond to ``integrable'' boundary configurations. We illustrate this with examples based on and , and comment on the relation of these D-branes to both non-Abelian T-duality and Poisson-Lie T-duality. We show that the D2 supported by D0 charge in the --deformed theory map, under analytic continuation together with Poisson-Lie T-duality, to D3 branes in the -deformation of the principal chiral model
Consenso mexicano sobre detección y tratamiento del cáncer gástrico incipiente
El cáncer gástrico representa una de las neoplasias más frecuentes en el aparato
digestivo y en la mayoría de los casos es el resultado de la progresión de lesiones premalignas.
La detección oportuna de estas lesiones es relevante ya que un tratamiento oportuno brinda
la posibilidad de curación. En nuestro país no existía un consenso respecto a la detección temprana del cáncer gástrico, por lo que la Asociación Mexicana de Gastroenterología reunió aun grupo de expertos y realizó el Consenso sobre detección y tratamiento del cáncer gástricoincipiente (CGI) para establecer recomendaciones de utilidad para la comunidad médica. Eneste consenso se utilizó la metodología Delphi y se emitieron 38 recomendaciones al respectodel CGI. El consenso define el CGI como aquel que al momento del diagnóstico se encuentralimitado a la mucosa y a la submucosa, independientemente de metástasis en ganglios linfáticos.En México, como otras partes del mundo, los factores asociados al CGI incluyen la infección porHelicobacter pylori, los antecedentes familiares, el tabaquismo y los factores dietéticos. Para eldiagnóstico se recomienda utilizar cromoendoscopia, magnificación y equipos con luz mejorada.Un diagnóstico histopatológico preciso es invaluable para tomar de decisiones terapéuticas. Eltratamiento endoscópico del CGI, ya sea disección o resección de la mucosa, debe ser preferidoal manejo quirúrgico cuando se puedan obtener resultados semejantes en términos de curaciónoncológica. La vigilancia endoscópica se deberá de individualizar
Distinct role of T helper Type 17 immune response for Graves\u27 hyperthyroidism in mice with different genetic backgrounds.
T helper type 17 (Th17) cells, a newly identified effector T-cell subset, have recently been shown to play a role in numerous autoimmune diseases, including iodine-induced autoimmune thyroiditis in non-obese diabetic (NOD)-H2(h4) mice, which had previously been thought Th1-dominant. We here studied the role of Th17 in Graves\u27 hyperthyroidism, another thyroid-specific autoimmune disease, in a mouse model. Two genetically distinct BALB/c and NOD-H2(h4) strains with intact or disrupted IL-17 genes (IL-17(+/+) or IL-17(-/-)) were immunized with adenovirus (Ad) expressing the thyrotropin receptor (TSHR) A-subunit (Ad-TSHR289). Both IL-17(+/+) and IL-17(-/-) mice developed anti-TSHR antibodies and hyperthyroidism at equally high frequencies on the BALB/c genetic background. In contrast, some IL-17(+/+), but none of IL-17(-/-), mice became hyperthyroid on the NOD-H2(h4) genetic background, indicating the crucial role of IL-17 for development of Graves\u27 hyperthyroidism in non-susceptible NOD-H2(h4), but not in susceptible BALB/c mice. In the T-cell recall assay, splenocytes and lymphocytes from the draining lymph nodes from either mouse strains, irrespective of IL-17 gene status, produced IFN-γ and IL-10 but not other cytokines including IL-17 in response to TSHR antigen. Thus, the functional significance of Th17 may not necessarily be predictable from cytokine expression patterns in splenocytes or inflammatory lesions. In conclusion, this is, to our knowledge, the first report showing that the role of Th17 cells for the pathogenesis of a certain autoimmune disease depends on the mouse genetic backgrounds
Genomics-assisted breeding in four major pulse crops of developing countries: present status and prospects
The global population is continuously increasing and is expected to reach nine billion by 2050. This huge population pressure will lead to severe shortage of food, natural resources and arable land. Such an alarming situation is most likely to arise in developing countries due to increase in the proportion of people suffering from protein and micronutrient malnutrition. Pulses being a primary and affordable source of proteins and minerals play a key role in alleviating the protein calorie malnutrition, micronutrient deficiencies and other undernourishment-related issues. Additionally, pulses are a vital source of livelihood generation for millions of resource-poor farmers practising agriculture in the semi-arid and sub-tropical regions. Limited success achieved through conventional breeding so far in most of the pulse crops will not be enough to feed the ever increasing population. In this context, genomics-assisted breeding (GAB) holds promise in enhancing the genetic gains. Though pulses have long been considered as orphan crops, recent advances in the area of pulse genomics are noteworthy, e.g. discovery of genome-wide genetic markers, high-throughput genotyping and sequencing platforms, high-density genetic linkage/QTL maps and, more importantly, the availability of whole-genome sequence. With genome sequence in hand, there is a great scope to apply genome-wide methods for trait mapping using association studies and to choose desirable genotypes via genomic selection. It is anticipated that GAB will speed up the progress of genetic improvement of pulses, leading to the rapid development of cultivars with higher yield, enhanced stress tolerance and wider adaptability
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