4,957 research outputs found
ABC likelihood-freee methods for model choice in Gibbs random fields
Gibbs random fields (GRF) are polymorphous statistical models that can be
used to analyse different types of dependence, in particular for spatially
correlated data. However, when those models are faced with the challenge of
selecting a dependence structure from many, the use of standard model choice
methods is hampered by the unavailability of the normalising constant in the
Gibbs likelihood. In particular, from a Bayesian perspective, the computation
of the posterior probabilities of the models under competition requires special
likelihood-free simulation techniques like the Approximate Bayesian Computation
(ABC) algorithm that is intensively used in population genetics. We show in
this paper how to implement an ABC algorithm geared towards model choice in the
general setting of Gibbs random fields, demonstrating in particular that there
exists a sufficient statistic across models. The accuracy of the approximation
to the posterior probabilities can be further improved by importance sampling
on the distribution of the models. The practical aspects of the method are
detailed through two applications, the test of an iid Bernoulli model versus a
first-order Markov chain, and the choice of a folding structure for two
proteins.Comment: 19 pages, 5 figures, to appear in Bayesian Analysi
Reactive Planar Manipulation with Convex Hybrid MPC
This paper presents a reactive controller for planar manipulation tasks that
leverages machine learning to achieve real-time performance. The approach is
based on a Model Predictive Control (MPC) formulation, where the goal is to
find an optimal sequence of robot motions to achieve a desired object motion.
Due to the multiple contact modes associated with frictional interactions, the
resulting optimization program suffers from combinatorial complexity when
tasked with determining the optimal sequence of modes.
To overcome this difficulty, we formulate the search for the optimal mode
sequences offline, separately from the search for optimal control inputs
online. Using tools from machine learning, this leads to a convex hybrid MPC
program that can be solved in real-time. We validate our algorithm on a planar
manipulation experimental setup where results show that the convex hybrid MPC
formulation with learned modes achieves good closed-loop performance on a
trajectory tracking problem
Spectral Efficiency Scaling Laws in Dense Random Wireless Networks with Multiple Receive Antennas
This paper considers large random wireless networks where
transmit-and-receive node pairs communicate within a certain range while
sharing a common spectrum. By modeling the spatial locations of nodes based on
stochastic geometry, analytical expressions for the ergodic spectral efficiency
of a typical node pair are derived as a function of the channel state
information available at a receiver (CSIR) in terms of relevant system
parameters: the density of communication links, the number of receive antennas,
the path loss exponent, and the operating signal-to-noise ratio. One key
finding is that when the receiver only exploits CSIR for the direct link, the
sum of spectral efficiencies linearly improves as the density increases, when
the number of receive antennas increases as a certain super-linear function of
the density. When each receiver exploits CSIR for a set of dominant interfering
links in addition to the direct link, the sum of spectral efficiencies linearly
increases with both the density and the path loss exponent if the number of
antennas is a linear function of the density. This observation demonstrates
that having CSIR for dominant interfering links provides a multiplicative gain
in the scaling law. It is also shown that this linear scaling holds for direct
CSIR when incorporating the effect of the receive antenna correlation, provided
that the rank of the spatial correlation matrix scales super-linearly with the
density. Simulation results back scaling laws derived from stochastic geometry.Comment: Submitte
Inter-Industry Wage Differentials and the Gender Wage Gap: Evidence from European Countries
This study analyses the interaction between inter-industry wage differentials and the gender wage gap in six European countries using a unique harmonised matched employeremployee data set, the 1995 European Structure of Earnings Survey. Findings show the existence of significant inter-industry wage differentials in all countries for both sexes. While their structure is quite similar for men and women and across countries, their dispersion is significantly larger in countries with decentralised bargaining. Further results indicate that industry effects on the gender wage gap fluctuate sharply across European countries. In particular, our results show that combined industry effects explain 29 per cent of the gender wage gap in Ireland.
Swelling of phospholipid floating bilayers: the effect of chain length
The equilibrium distance between two lipid bilayers stable in bulk water and
in proximity of a substrate was investigated. Samples consisted of a
homogeneous lipid bilayer, floating near an identical bilayer deposited on the
hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl
phosphocholines, with the number of carbon atoms per chain, n, varying from 16
to 20. The average and r.m.s. positions of the floating bilayer were determined
by means of neutron specular reflectivity. Samples were prepared at room
temperature (i.e. with the lipids in the gel phase) and measurements performed
at various temperatures so that the whole region of transition from gel to
fluid phase was explored. Data have been interpreted in terms of competition
between the interbilayer potential and membrane fluctuations and used to
estimate the bending rigidity of the bilayer
Role of non-ideality for the ion transport in porous media: derivation of the macroscopic equations using upscaling
This paper is devoted to the homogenization (or upscaling) of a system of
partial differential equations describing the non-ideal transport of a
N-component electrolyte in a dilute Newtonian solvent through a rigid porous
medium. Realistic non-ideal effects are taken into account by an approach based
on the mean spherical approximation (MSA) model which takes into account finite
size ions and screening effects. We first consider equilibrium solutions in the
absence of external forces. In such a case, the velocity and diffusive fluxes
vanish and the equilibrium electrostatic potential is the solution of a variant
of Poisson-Boltzmann equation coupled with algebraic equations. Contrary to the
ideal case, this nonlinear equation has no monotone structure. However, based
on invariant region estimates for Poisson-Boltzmann equation and for small
characteristic value of the solute packing fraction, we prove existence of at
least one solution. To our knowledge this existence result is new at this level
of generality. When the motion is governed by a small static electric field and
a small hydrodynamic force, we generalize O'Brien's argument to deduce a
linearized model. Our second main result is the rigorous homogenization of
these linearized equations and the proof that the effective tensor satisfies
Onsager properties, namely is symmetric positive definite. We eventually make
numerical comparisons with the ideal case. Our numerical results show that the
MSA model confirms qualitatively the conclusions obtained using the ideal model
but there are quantitative differences arising that can be important at high
charge or high concentrations.Comment: 46 page
Implementing the GPRA: Examining the Prospects for Performance Budgeting in the Federal Government
The Government Performance and Results Act (GPRA) is the latest in a series
of attempts to introduce performance-based management and budgeting
techniques at the federal level in the United States. In the past, these attempts
largely failed due to administrative complexities, lack of investment in
managerial, accounting, and information systems, and the absence of
institutional incentives to promote gains in economic efficiency. Whereas we
find the objectives of the GPRA laudable, we question whether this current
incarnation of performance budgeting can succeed in transforming the
traditional focus of federal budgeting from annual appropriations and
obligations to multiyear outputs and outcomes
Migrations et recherches en Allemagne Fédérale
Hettlage Robert, Khim Francois. Migrations et recherches en Allemagne Fédérale. In: Revue européenne des migrations internationales, vol. 2, n°2, Novembre 1986. Amériques, sous la direction de Yves Charbit et Jacqueline Costa-Lascoux. pp. 165-192
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