10,234 research outputs found
Sharpness versus robustness of the percolation transition in 2D contact processes
We study versions of the contact process with three states, and with
infections occurring at a rate depending on the overall infection density.
Motivated by a model described in [17] for vegetation patterns in arid
landscapes, we focus on percolation under invariant measures of such processes.
We prove that the percolation transition is sharp (for one of our models this
requires a reasonable assumption). This is shown to contradict a form of
'robust critical behaviour' with power law cluster size distribution for a
range of parameter values, as suggested in [17].Comment: 31 pages, to appear in Stochastic Processes and their Application
Thruster Allocation for Dynamical Positioning
Positioning a vessel at a fixed position in deep water is of great importance when working offshore. In recent years a Dynamical Positioning (DP) system was developed at Marin [2]. After the measurement of the current position and external forces (like waves, wind etc.), each thruster of the vessel is actively controlled to hold the desired location.
In this paper we focus on the allocation process to determine the settings for each thruster that results in the minimal total power and thus fuel consumption. The mathematical formulation of this situation leads to a nonlinear optimization problem with equality and inequality constraints, which can be solved by applying Lagrange multipliers.
We give three approaches: first of all, the full problem was solved using the MATLAB fmincon routine with the solution from the linearised problem as a starting point. This implementation, with robust handling of the situations where the thrusters are overloaded, lead to promising results: an average reduction in fuel consumption of approximately two percent. However, further analysis proved useful. A second approach changes the set of variables and so reduces the number of equations. The third and last approach solves the Lagrange equations with an iterative method on the linearized Lagrange problem
Crosscutting, what is and what is not? A Formal definition based on a Crosscutting Pattern
Crosscutting is usually described in terms of scattering and tangling. However, the distinction between these concepts is vague, which could lead to ambiguous statements. Sometimes, precise definitions are required, e.g. for the formal identification of crosscutting concerns. We propose a conceptual framework for formalizing these concepts based on a crosscutting pattern that shows the mapping between elements at two levels, e.g. concerns and representations of concerns. The definitions of the concepts are formalized in terms of linear algebra, and visualized with matrices and matrix operations. In this way, crosscutting can be clearly distinguished from scattering and tangling. Using linear algebra, we demonstrate that our definition generalizes other definitions of crosscutting as described by Masuhara & Kiczales [21] and Tonella and Ceccato [28]. The framework can be applied across several refinement levels assuring traceability of crosscutting concerns. Usability of the framework is illustrated by means of applying it to several areas such as change impact analysis, identification of crosscutting at early phases of software development and in the area of model driven software development
The Canonical Perfect Bose Gas in Casimir Boxes
We study the problem of Bose-Einstein condensation in the perfect Bose gas in
the canonical ensemble, in anisotropically dilated rectangular parallelpipeds
(Casimir boxes). We prove that in the canonical ensemble for these anisotropic
boxes there is the same type of generalized Bose-Einstein condensation as in
the grand-canonical ensemble for the equivalent geometry. However the amount of
condensate in the individual states is different in some cases and so are the
fluctuations.Comment: 23 page
Localized states influence spin transport in epitaxial graphene
We developed a spin transport model for a diffusive channel with coupled
localized states that result in an effective increase of spin precession
frequencies and a reduction of spin relaxation times in the system. We apply
this model to Hanle spin precession measurements obtained on monolayer
epitaxial graphene on SiC(0001) (MLEG). Combined with newly performed
measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001)
our analysis shows that the different values for the diffusion coefficient
measured in charge and spin transport measurements in MLEG and the high values
for the spin relaxation time can be explained by the influence of localized
states arising from the buffer layer at the interface between the graphene and
the SiC surface.Comment: 6 pages, 3 figures, including supplementary materia
Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field
We studied the stellar population in the central 6.6x6.6arcmin,region of the
ultra-deep (1Msec) Chandra Galactic field - the "Chandra bulge field" (CBF)
approximately 1.5 degrees away from the Galactic Center - using the Hubble
Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on
the behavior of red clump giants - a distinct stellar population, which is
known to have an essentially constant intrinsic luminosity and color. By
studying the variation in the position of the red clump giants on a spatially
resolved color-magnitude diagram, we confirm the anomalous total-to-selective
extinction ratio, as reported in previous work for other Galactic bulge fields.
We show that the interstellar extinction in this area is = 4 on
average, but varies significantly between ~3-5 on angular scales as small as 1
arcminute. Using the distribution of red clump giants in an
extinction-corrected color-magnitude diagram, we constrain the shape of a
stellar-mass distribution model in the direction of this ultra-deep Chandra
field, which will be used in a future analysis of the population of X-ray
sources. We also show that the adopted model for the stellar density
distribution predicts an infrared surface brightness in the direction of the
"Chandra bulge field" in good agreement (i.e. within ~15%) with the actual
measurements derived from the Spitzer/IRAC observations.Comment: 9 pages, 9 figures. Accepted for publication in A&
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