5,142 research outputs found
Anomalous bulk behaviour in the free parafermion spin chain
We demonstrate using direct numerical diagonalization and extrapolation
methods that boundary conditions have a profound effect on the bulk properties
of a simple model for for which the model hamiltonian is
non-hermitian. For the model reduces to the well known quantum Ising
model in a transverse field. For open boundary conditions the model is
known to be solved exactly in terms of free parafermions. Once the ends of the
open chain are connected by considering the model on a ring, the bulk
properties, including the ground-state energy per site, are seen to differ
dramatically with increasing . Other properties, such as the leading
finite-size corrections to the ground-state energy, the mass gap exponent and
the specific heat exponent, are also seen to be dependent on the boundary
conditions. We speculate that this anomalous bulk behaviour is a topological
effect.Comment: 8 pages, 8 figures, minor change
A sequential quadratic programming algorithm using an incomplete solution of the subproblem
We analyze sequential quadratic programming (SQP) methods to solve nonlinear
constrained optimization problems that are more flexible in their definition than standard SQP
methods. The type of flexibility introduced is motivated by the necessity to deviate from the standard
approach when solving large problems. Specifically we no longer require a minimizer of the QP
subproblem to be determined or particular Lagrange multiplier estimates to be used. Our main focus
is on an SQP algorithm that uses a particular augmented Lagrangian merit function. New results
are derived for this algorithm under weaker conditions than previously assumed; in particular, it is
not assumed that the iterates lie on a compact setThis research was supported by National Science Foundation grant DDMo9204208, Department
of Energy grant DE-FG03-92ER25117, Office of Naval Research grant N00014-90-J-1242, and the
Bank of SpainPublicad
Interacción de controles humanos y ambientales sobre la ecologÃa y socio - economÃa de sistemas productivos semiáridos del Chaco y Espinal
El avance agrÃcola hacia regiones de mayor aridez crece en todo el mundo y particularmente en Sudamérica, generando cuestionamientos sobre su viabilidad productiva, sus costos ambientales y beneficios socio-económicos. Ante estos cuestionamientos surge la necesidad de comparar los principales sistemas agrÃcolas semiáridos y las alternativas predominantes de uso del territorio, considerando múltiples perspectivas que abarquen sucesivamente el plano ecológico, agronómico, económico y social. A escala de lote, compromisos biológicos entre la capacidad de tolerar la aridez y sostener altas productividades sugieren la existencia de rangos óptimos en gradientes de lluvia, en los que sucesivamente resultan ventajosos los cultivos anuales, las pasturas perennes y la vegetación natural. Se exploraron los resultados para los principales usos (forestal, ganadero y agrÃcola) y coberturas (bosque, pasturas C4, maÃz, soja y trigo) del Chaco y Espinal semiárido (900 a 400 mm y 14 a 25 °C). El cultivo de maÃz duplicó la producción de biomasa y alimento de las demás coberturas en todo el gradiente ambiental. AsÃ, la elección del tipo de cultivo excederÃa el efecto del tipo de uso definiendo estos dos atributos ecológicos y agronómicos fundamentales. Menores costos de transporte y semilla o su transformación local revertirÃan su menor renta respecto a la soja, que actualmente prevalece por su mayor precio a pesar de su menor productividad. El rinde y la renta del uso forestal tradicional de la vegetación natural resultó extremadamente bajo, incluso maximizando la cosecha de madera y su precio, dificultando su conservación. Disminuir esta brecha demandarÃa mecanizar eficientemente usos alternativos compatibles con el bosque, como silvopastura o cosecha de bioenergÃa. Notablemente, no existió un nicho óptimo para cultivos en el extremo húmedo ni para pasturas en el árido, respecto a su adaptación, productividad e impacto ecológico. Por ende, maximizar la actividad fotosintética, transpiración y conservación de servicios del ecosistema, requerirÃa estrategias mixtas que combinen y alternen cultivos de renta con otros de cobertura y pasturas con especies leñosas. A escala regional, las complejas interacciones entre estrategias productivas y de distribución de recursos, sugieren una vinculación no lineal entre los factores sucesivos que contribuyen al bienestar humano. Se compararon dos fronteras agrÃcolas del Chaco semiárido, biofÃsicamente similares pero con desarrollos contrastantes (actores locales organizados vs inversores externos). El beneficio socio-económico del ahorro en transporte y agregado de valor mediante agroindustria local excedió el de maximizar el rinde o renta a escala de parcela, potenciando usos ecológicamente menos disruptivos. El rol clave de la capacidad de organización y gestión en el desarrollo cuestiona enfoques netamente agronómicos que no incorporan aspectos polÃtico-organizativos a los objetivos principales de intervención e investigación
Complexity and anisotropy in host morphology make populations safer against epidemic outbreaks
One of the challenges in epidemiology is to account for the complex
morphological structure of hosts such as plant roots, crop fields, farms,
cells, animal habitats and social networks, when the transmission of infection
occurs between contiguous hosts. Morphological complexity brings an inherent
heterogeneity in populations and affects the dynamics of pathogen spread in
such systems. We have analysed the influence of realistically complex host
morphology on the threshold for invasion and epidemic outbreak in an SIR
(susceptible-infected-recovered) epidemiological model. We show that disorder
expressed in the host morphology and anisotropy reduces the probability of
epidemic outbreak and thus makes the system more resistant to epidemic
outbreaks. We obtain general analytical estimates for minimally safe bounds for
an invasion threshold and then illustrate their validity by considering an
example of host data for branching hosts (salamander retinal ganglion cells).
Several spatial arrangements of hosts with different degrees of heterogeneity
have been considered in order to analyse separately the role of shape
complexity and anisotropy in the host population. The estimates for invasion
threshold are linked to morphological characteristics of the hosts that can be
used for determining the threshold for invasion in practical applications.Comment: 21 pages, 8 figure
Agglomerate Behaviour in a Recirculating Fluidized Bed with Sheds: Effect of Agglomerate Properties
Crystallization and melting of bacteria colonies and Brownian Bugs
Motivated by the existence of remarkably ordered cluster arrays of bacteria
colonies growing in Petri dishes and related problems, we study the spontaneous
emergence of clustering and patterns in a simple nonequilibrium system: the
individual-based interacting Brownian bug model. We map this discrete model
into a continuous Langevin equation which is the starting point for our
extensive numerical analyses. For the two-dimensional case we report on the
spontaneous generation of localized clusters of activity as well as a
melting/freezing transition from a disordered or isotropic phase to an ordered
one characterized by hexagonal patterns. We study in detail the analogies and
differences with the well-established Kosterlitz-Thouless-Halperin-Nelson-Young
theory of equilibrium melting, as well as with another competing theory. For
that, we study translational and orientational correlations and perform a
careful defect analysis. We find a non standard one-stage, defect-mediated,
transition whose nature is only partially elucidated.Comment: 13 Figures. 14 pages. Submitted to Phys. Rev.
Legumes display common and host-specific responses to the rhizobial cellulase CelC2 during primary symbiotic infection.
Primary infection of legumes by rhizobia involves the controlled localized enzymatic breakdown of cell walls at root hair tips. Previous studies determined the role of rhizobial CelC2 cellulase in different steps of the symbiotic interaction Rhizobium leguminosarum-Trifolium repens. Recent findings also showed that CelC2 influences early signalling events in the Ensifer meliloti-Medicago truncatula interaction. Here, we have monitored the root hair phenotypes of two legume plants, T. repens and M. sativa, upon inoculation with strains of their cognate and non-cognate rhizobial species, R. leguminosarum bv trifolii and E. meliloti, (over)expressing the CelC2 coding gene, celC. Regardless of the host, CelC2 specifically elicited ‘hole-on-the-tip’ events (Hot phenotype) in the root hair apex, consistent with the role of this endoglucanase in eroding the noncrystalline cellulose found in polarly growing cell walls. Overproduction of CelC2 also increased root hair tip redirections (RaT phenotype) events in both cognate and non-cognate hosts. Interestingly, heterologous celC expression also induced non-canonical alterations in ROS (Reactive Oxygen Species) homeostasis at root hair tips of Trifolium and Medicago. These results suggest the concurrence of shared unspecific and host-related plant responses to CelC2 during early steps of symbiotic rhizobial infection. Our data thus identify CelC2 cellulase as an important determinant of events underlying early infection of the legume host by rhizobia
- …