636 research outputs found
Exact solutions in a scalar-tensor model of dark energy
We consider a model of scalar field with non minimal kinetic and Gauss Bonnet
couplings as a source of dark energy. Based on asymptotic limits of the
generalized Friedmann equation, we impose restrictions on the kinetic an
Gauss-Bonnet couplings. This restrictions considerable simplify the equations,
allowing for exact solutions unifying early time matter dominance with
transitions to late time quintessence and phantom phases. The stability of the
solutions in absence of matter has been studied.Comment: 30 pages, 2 figures, to appear in JCA
On the Ricci dark energy model
We study the Ricci dark energy model (RDE) which was introduced as an
alternative to the holographic dark energy model. We point out that an
accelerating phase of the RDE is that of a constant dark energy model. This
implies that the RDE may not be a new model of explaining the present
accelerating universe.Comment: 8 page
Dynamic longitudinal stability and control of tandem coupled bomber-fighter airplane models with rigid and pitch-free coupling
Credit and saving constraints in general equilibrium : evidence from survey data
RESUMEN: En este documento, construimos un modelo de equilibrio general dinĂĄmico de agentes heterogĂ©neos en el que las restricciones de ahorro interactĂșan con las restricciones de crĂ©dito. Las restricciones de ahorro en forma de costos fijos para usar el sistema financiero llevan a los hogares a buscar instrumentos de ahorro informales (efectivo) y a un menor ahorro agregado. Las restricciones crediticias inducen una mala asignaciĂłn de capital entre los productores, lo que a su vez reduce el producto, la productividad y el rendimiento de los instrumentos financieros formales. Calibramos el modelo utilizando datos de encuestas de un paĂs en desarrollo donde las restricciones informales de ahorro y crĂ©dito son generalizadas. Nuestros resultados cuantitativos sugieren que eliminar por completo las restricciones de ahorro y crĂ©dito puede tener grandes efectos sobre las tasas de ahorro, la producciĂłn, la PTF y el bienestar.ABSTRACT: In this paper, we build a heterogeneous agents-dynamic general equilibrium model wherein saving constraints interact with credit constraints. Saving constraints in the form of fixed costs to use the financial system lead households to seek informal saving instruments (cash) and result in lower aggregate saving. Credit constraints induce misallocation of capital across
producers that in turn lowers output, productivity, and the return to formal financial instruments. We calibrate the model using survey data from a developing country where informal saving and credit constraints are pervasive. Our quantitative results suggest that completely removing saving and credit constraints can have large effects on saving rates, output, TFP,
and welfare. Moreover, we note that a sizable fraction of these gains can be more easily attained by a mix of moderate reforms that lower both types of frictions than by a strong reform on either front
Erratum to: âEffectiveness of a training program in compliance with recommendations for venous lines careâ
Reconstructing the potentials for the quintessence and tachyon dark energy, from the holographic principle
We propose an holographic quintessence and tachyon models of dark energy. The
correspondence between the quintessence and tachyon energy densities with the
holographic density, allows the reconstruction of the potentials and the
dynamics for the quintessence and tachyon fields, in flat FRW background. The
proposed infrared cut-off for the holographic energy density works for two
cases of the constant : for we reconstructed the holographic
quintessence model in the region before the crossing for the EoS
parameter. The cosmological dynamics for was also reconstructed for
the holographic quintessence and tachyon models.Comment: 21 pages, 18 figures, 2 table
Super-resolution, Extremal Functions and the Condition Number of Vandermonde Matrices
Super-resolution is a fundamental task in imaging, where the goal is to
extract fine-grained structure from coarse-grained measurements. Here we are
interested in a popular mathematical abstraction of this problem that has been
widely studied in the statistics, signal processing and machine learning
communities. We exactly resolve the threshold at which noisy super-resolution
is possible. In particular, we establish a sharp phase transition for the
relationship between the cutoff frequency () and the separation ().
If , our estimator converges to the true values at an inverse
polynomial rate in terms of the magnitude of the noise. And when no estimator can distinguish between a particular pair of
-separated signals even if the magnitude of the noise is exponentially
small.
Our results involve making novel connections between {\em extremal functions}
and the spectral properties of Vandermonde matrices. We establish a sharp phase
transition for their condition number which in turn allows us to give the first
noise tolerance bounds for the matrix pencil method. Moreover we show that our
methods can be interpreted as giving preconditioners for Vandermonde matrices,
and we use this observation to design faster algorithms for super-resolution.
We believe that these ideas may have other applications in designing faster
algorithms for other basic tasks in signal processing.Comment: 19 page
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