5,767 research outputs found
A canonical space-time state space model: state and parameter estimation
The maximum likelihood estimation of a dynamic spatiotemporal model is introduced, centred around the inclusion of a prior arbitrary spatiotemporal neighborhood description. The neighborhood description defines a specific parameterization of the state transition matrix, chosen on the basis of prior knowledge about the system. The model used is inspired by the spatiotemporal ARMA (STARMA) model, but the representation used is based on the standard state-space model. The inclusion of the neighborhood into an expectation-maximization based joint state and parameter estimation algorithm allows for accurate characterization of the spatiotemporal model. The process of including the neighborhood, and the effect it has on the maximum likelihood parameter estimate is described and demonstrated in this paper
Optimizing Omega
"The original publication is available at www.springerlink.com " Copyright Springer. DOI: 10.1007/s10898-008-9396-5This paper considers the Omega function, proposed by Cascon, Keating & Shadwick as a performance measure for comparing financial assets. We discuss the use of Omega as a basis for portfolio selection. We show that the problem of choosing portfolio weights in order to maximize Omega typically has many local solutions and we describe some preliminary computational experience of finding the global optimum using a NAG library implementation of the Huyer & Neumaier MCS method.Peer reviewe
MHD Memes
The celebration of Allan Kaufman's 80th birthday was an occasion to reflect
on a career that has stimulated the mutual exchange of ideas (or memes in the
terminology of Richard Dawkins) between many researchers. This paper will
revisit a meme Allan encountered in his early career in magnetohydrodynamics,
the continuation of a magnetohydrodynamic mode through a singularity, and will
also mention other problems where Allan's work has had a powerful
cross-fertilizing effect in plasma physics and other areas of physics and
mathematics.Comment: Submitted for publication in IOP Journal of Physics: Conference
Series for publication in "Plasma Theory, Wave Kinetics, and Nonlinear
Dynamics", Proceedings of KaufmanFest, 5-7 October 2007, University of
California, Berkeley, US
A Future Teachers Conference – A Vehicle to Retain, Inform, and Inspire New and Prospective Teachers
The Los Angeles Collaborative for Teacher Excellence (LACTE) Future Teachers Conference is a day-long learning event for beginning and pre-service K-12 math and science teachers. The Conference provides information, resources and connections tailored to the needs of prospective and new teachers. A unique aspect of the conference is that a group of new and future teachers serve as partners with college faculty on the planning committee. Thus the Conference not only serves to educate new and future teachers, but also provides leadership training for the planning team members; and, the presence of prospective and new teachers on the planning team ensures that the conference sessions effectively target the intended audience\u27s interests. This conference is the most popular activity for new and prospective teachers in the Collaborative. Over the five years the conference has been held, its attendance has grown to 120-150 participants annually. A planning handbook is available to assist anyone interested in organizing a similar event
Parameter estimation and inference for stochastic reaction-diffusion systems: application to morphogenesis in D. melanogaster
Background: Reaction-diffusion systems are frequently used in systems biology to model developmental and signalling processes. In many applications, count numbers of the diffusing molecular species are very low, leading to the need to explicitly model the inherent variability using stochastic methods. Despite their importance and frequent use, parameter estimation for both deterministic and stochastic reaction-diffusion systems is still a challenging problem.
Results: We present a Bayesian inference approach to solve both the parameter and state estimation problem for stochastic reaction-diffusion systems. This allows a determination of the full posterior distribution of the parameters (expected values and uncertainty). We benchmark the method by illustrating it on a simple synthetic experiment. We then test the method on real data about the diffusion of the morphogen Bicoid in Drosophila melanogaster. The results show how the precision with which parameters can be inferred varies dramatically, indicating that the ability to infer full posterior distributions on the parameters can have important experimental design consequences.
Conclusions: The results obtained demonstrate the feasibility and potential advantages of applying a Bayesian approach to parameter estimation in stochastic reaction-diffusion systems. In particular, the ability to estimate credibility intervals associated with parameter estimates can be precious for experimental design. Further work, however, will be needed to ensure the method can scale up to larger problems
Nonaxisymmetric, multi-region relaxed magnetohydrodynamic equilibrium solutions
We describe a magnetohydrodynamic (MHD) constrained energy functional for
equilibrium calculations that combines the topological constraints of ideal MHD
with elements of Taylor relaxation.
Extremizing states allow for partially chaotic magnetic fields and
non-trivial pressure profiles supported by a discrete set of ideal interfaces
with irrational rotational transforms.
Numerical solutions are computed using the Stepped Pressure Equilibrium Code,
SPEC, and benchmarks and convergence calculations are presented.Comment: Submitted to Plasma Physics and Controlled Fusion for publication
with a cluster of papers associated with workshop: Stability and Nonlinear
Dynamics of Plasmas, October 31, 2009 Atlanta, GA on occasion of 65th
birthday of R.L. Dewar. V2 is revised for referee
Generalised action-angle coordinates defined on island chains
Straight-field-line coordinates are very useful for representing magnetic
fields in toroidally confined plasmas, but fundamental problems arise regarding
their definition in 3-D geometries because of the formation of islands and
chaotic field regions, ie non-integrability. In Hamiltonian dynamical systems
terms these coordinates are a form of action-angle variables, which are
normally defined only for integrable systems. In order to describe 3-D magnetic
field systems, a generalisation of this concept was proposed recently by the
present authors that unified the concepts of ghost surfaces and
quadratic-flux-minimising (QFMin) surfaces. This was based on a simple
canonical transformation generated by a change of variable , where and are poloidal and toroidal
angles, respectively, with a new poloidal angle chosen to give
pseudo-orbits that are a) straight when plotted in the plane and
b) QFMin pseudo-orbits in the transformed coordinate. These two requirements
ensure that the pseudo-orbits are also c) ghost pseudo-orbits. In the present
paper, it is demonstrated that these requirements do not \emph{uniquely}
specify the transformation owing to a relabelling symmetry. A variational
method of solution that removes this lack of uniqueness is proposed.Comment: 10 pages. Accepted by Plasma Physics and Controlled Fusion as part of
a cluster of refereed papers in a special issue containing papers arising
from the Joint International Stellarator & Heliotron Workshop and
Asia-Pacific Plasma Theory Conference, held in Canberra and Murramarang
Resort, Australia, 30 January - 3 February, 201
Opportunities for weed manipulation using GMHT row crops
The herbicides and cultivation systems available in most non-GM crops allow farmers little flexibility as to when they control weeds. However, glyphosate and glufosinate-ammonium, as used in GM herbicide tolerant crops, offer the opportunity to control large weeds and weed control can be timed according to the agronomic and environmental aims of the user. This paper will use sugar beet as a model crop and report results where different approaches to weed control have been used and discuss their relevance in the wider agricultural and environmental contextNon peer reviewe
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