1,817 research outputs found
The Fractional Ornstein-Uhlenbeck Process: Term Structure Theory and Application
The paper revisits dynamic term structure models (DTSMs) and proposes a new way in dealing with the limitation of the classical affine models. In particular, this paper expands the flexibility of the DTSMs by applying a fractional Brownian motion as the governing force of the state variable instead of the standard Brownian motion. This is a new direction in pricing non defaultable bonds with offspring in the arbitrage free pricing of weather derivatives based on fractional Brownian motions. By applying fractional Ito calculus and a fractional version of the Girsanov transform, a no arbitrage price of the bond is recovered by solving a fractional version of the fundamental bond pricing equation. Besides this theoretical contribution, the paper proposes an estimation methodology based on the Kalman filter approach, which is applied to the US term structure of interest rates.Fractional bond pricing equation; fractional Brownian motion; fractional Ornstein-Uhlenbeck process; long memory; Kalman filter
Estimating true age-dependence in survival when only adults can be observed: an example with Black-legged Kittiwakes
In long-lived birds, pre-breeders are often difficult or impossible to observe, and even though a proportion of marked adults may be of known age, the estimation of age-specific survival is complicated by the absence of observations during the first years of life. New developments in MARK now allow use of an updated individual covariate. We used this powerful approach to model age-dependence in survival of Black-legged Kittiwakes (Rissa tridactyla) at a North Sea colony. Although only 69 marked breeders were of known age, there was strong evidence for a quadratic relationship between true age and survival. We believe that this simple but powerful approach could be implemented for many species and could provide improved estimates of how survival changes with age, a central theme in life history theory
CĂłmo estimar el efecto real de la edad verdadera en la supervivencia cuando sĂłlo es posible observar individuos adultos: ejemplo de la gaviota tridĂĄctila
In longâlived birds, preâbreeders are often difficult or impossible to observe, and even though a proportion of marked adults may be of known age, the estimation of ageâspecific survival is complicated by the absence of observations during the first years of life. New developments in MARK now allow use of an updated individual covariate. We used this powerful approach to model ageâdependence in survival of Blackâlegged Kittiwakes (Rissa tridactyla) at a North Sea colony. Although only 69 marked breeders were of known age, there was strong evidence for a quadratic relationship between true age and survival. We believe that this simple but powerful approach could be implemented for many species and could provide improved estimates of how survival changes with age, a central theme in life history theory.En las aves de larga vida, a menudo resulta difĂcil o incluso imposible observar individuos prerreproductores, y si bien un porcentaje de adultos marcados pueden ser de edad conocida, la estimaciĂłn de la supervivencia a una edad especĂfica se convierte en una tarea compleja, puesto que no se dispone de observaciones de los primeros aĂąos de vida. Las nuevas caracterĂsticas del programa MARK nos permiten utilizar una covarianza individual actualizada. Empleamos este impactante enfoque para modelizar el efecto de la edad en la supervivencia de la gaviota tridĂĄctila (Rissa tridactyla) en una colonia del mar del Norte. Aunque sĂłlo 69 aves reproductoras marcadas eran de edad conocida, contĂĄbamos con numerosas pruebas que apuntaban a una relaciĂłn cuadrĂĄtica entre la edad real y la supervivencia. Creemos que este simple pero eficaz enfoque podrĂa aplicarse en muchas especies, proporcionando estimaciones mejoradas acerca de cĂłmo la supervivencia varĂa con la edad, un tema central de la teorĂa de las historias vitales
A technique for extracting potentially predictable patterns from climate data
We propose a computational technique which makes it possible to extract long-range potentially predictable patterns of interannual variability of meteorological seasonal mean fields. These patterns arise from slowly varying external forcing, such as sea surface temperatures, and slowly varying internal dynamics. The method provides a means of decomposing the covariance matrix of a seasonal mean field into covariance matrices for the potentially predictable and the chaotic, or weather-noise, components, separately. We illustrate the technique using Australian surface maximum temperatures during December-January-February (DJF) for the period 1958--1991. The dominant patterns, arising from the potentially predictable covariance matrix, are shown to be more closely related to slowly varying external forcing and slowly varying internal dynamics than those from a conventional analysis. The importance of tropical sea surface temperatures in forcing the potentially predictable patterns is also discussed
Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find small differences in the results for no ambient and
modest ambient magnetic fields. Simulations show that the Weibel instability
created in the collisionless shock front accelerates jet and ambient particles
both perpendicular and parallel to the jet propagation direction. The small
scale magnetic field structure generated by the Weibel instability is
appropriate to the generation of ``jitter'' radiation from deflected electrons
(positrons) as opposed to synchrotron radiation. The jitter radiation resulting
from small scale magnetic field structures may be important for understanding
the complex time structure and spectral evolution observed in gamma-ray bursts
or other astrophysical sources containing relativistic jets and relativistic
collisionless shocks.Comment: 6 pages, 1 figure, revised and accepted for Advances in Space
Research (35th COSPAR Scientific Assembly, Paris, 18-25 July 2004
Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: from fluid to kinetic modeling
The nonlinear evolution of collisionless plasmas is typically a multi-scale
process where the energy is injected at large, fluid scales and dissipated at
small, kinetic scales. Accurately modelling the global evolution requires to
take into account the main micro-scale physical processes of interest. This is
why comparison of different plasma models is today an imperative task aiming at
understanding cross-scale processes in plasmas. We report here the first
comparative study of the evolution of a magnetized shear flow, through a
variety of different plasma models by using magnetohydrodynamic, Hall-MHD,
two-fluid, hybrid kinetic and full kinetic codes. Kinetic relaxation effects
are discussed to emphasize the need for kinetic equilibriums to study the
dynamics of collisionless plasmas in non trivial configurations. Discrepancies
between models are studied both in the linear and in the nonlinear regime of
the magnetized Kelvin-Helmholtz instability, to highlight the effects of small
scale processes on the nonlinear evolution of collisionless plasmas. We
illustrate how the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field direction
during the linear evolution when kinetic effects are taken into account. Even
if we found that small scale processes differ between the different models, we
show that the feedback from small, kinetic scales to large, fluid scales is
negligable in the nonlinear regime. This study show that the kinetic modeling
validates the use of a fluid approach at large scales, which encourages the
development and use of fluid codes to study the nonlinear evolution of
magnetized fluid flows, even in the colisionless regime
Bayesian Error Estimation in Density Functional Theory
We present a practical scheme for performing error estimates for Density
Functional Theory calculations. The approach which is based on ideas from
Bayesian statistics involves creating an ensemble of exchange-correlation
functionals by comparing with an experimental database of binding energies for
molecules and solids. Fluctuations within the ensemble can then be used to
estimate errors relative to experiment on calculated quantities like binding
energies, bond lengths, and vibrational frequencies. It is demonstrated that
the error bars on energy differences may vary by orders of magnitude for
different systems in good agreement with existing experience.Comment: 5 pages, 3 figure
The Construction and Validation of an Arithmetical Computation Test
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67909/2/10.1177_001316445301300206.pd
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