701 research outputs found
An Information-Based Neural Approach to Constraint Satisfaction
A novel artificial neural network approach to constraint satisfaction
problems is presented. Based on information-theoretical considerations, it
differs from a conventional mean-field approach in the form of the resulting
free energy. The method, implemented as an annealing algorithm, is numerically
explored on a testbed of K-SAT problems. The performance shows a dramatic
improvement to that of a conventional mean-field approach, and is comparable to
that of a state-of-the-art dedicated heuristic (Gsat+Walk). The real strength
of the method, however, lies in its generality -- with minor modifications it
is applicable to arbitrary types of discrete constraint satisfaction problems.Comment: 13 pages, 3 figures,(to appear in Neural Computation
Titrating Polyelectrolytes - Variational Calculations and Monte Carlo Simulations
Variational methods are used to calculate structural and thermodynamical
properties of a titrating polyelectrolyte in a discrete representation. The
Coulomb interactions are emulated by harmonic repulsive forces, the force
constants being used as variational parameters to minimize the free energy. For
the titrating charges, a mean field approach is used.
The accuracy is tested against Monte Carlo data for up to 1000 monomers. For
an unscreened chain, excellent agreement is obtained for the end-to-end
distance and the apparent dissociation constant. With screening, the
thermodynamical properties are invariably well described, although the
structural agreement deteriorates.
A very simple rigid-rod approximation is also considered, giving surprisingly
good results for certain properties.Comment: 22 pages, PostScript, 9 figure
An information-based neural approach to generic constraint satisfaction
AbstractA novel artificial neural network heuristic (INN) for general constraint satisfaction problems is presented, extending a recently suggested method restricted to boolean variables. In contrast to conventional ANN methods, it employs a particular type of non-polynomial cost function, based on the information balance between variables and constraints in a mean-field setting. Implemented as an annealing algorithm, the method is numerically explored on a testbed of Graph Coloring problems. The performance is comparable to that of dedicated heuristics, and clearly superior to that of conventional mean-field annealing
Charge regulation in biomolecular solution.
Proteins and other biomolecules contain acidic and basic titratable groups that give rise to intricate charge distributions and control electrostatic interactions. 'Charge regulation' concerns how the proton equilibria of these sites are perturbed when approached by alien molecular matter such as other proteins, surfaces and membranes, DNA, polyelectrolytes etc. Importantly, this perturbation generates a charge response that leads to attractive intermolecular interactions that can be conveniently described by a single molecular property - the charge capacitance. The capacitance quantifies molecular charge fluctuations, i.e. it is the variance of the mean charge and is an intrinsic property on par with the net charge and the dipole moment. It directly enters the free energy expression for intermolecular interactions and can be obtained experimentally from the derivative of the titration curve or theoretically from simulations. In this review, we focus on the capacitance concept as a predictive parameter for charge regulation and demonstrate how it can be used to estimate the interaction of a protein with other proteins, polyelectrolytes, membranes as well as with ligands
sf 36 scores in degenerative lumbar spine disorders analysis of prospective data from 451 patients
Background When using Health-Related Quality of Life (HRQoL) in assessing outcomes of treatment, normative data for different diagnoses are needed to allow cross-comparisons of existing and future studies. We determined the SF-36 scores in patients with surgical lumbar spine problems.Methods This is a prospective observational study of consecutive surgical patients in one institution. In addition to SF-36 questionnaire responses, local pain, radiating pain, analgesic intake and walking ability were recorded, together with several other demographic variables. 451 patients (50% women) with median age 52 (13–88) years, operated from 1998 through 2002, were included in the study.Results Preoperative SF-36 scores were significantly lower than those derived from previously published material (the general population, nonspecific low back pain (LBP) patients, other samples of non-LBP patients), also with the use of normbased scoring. Sick-leave and worker's compensation seemed to affect perceived Health-Related Q..
The equation of state of colloidal dispersions.
International audienceWe present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the Primitive Model combined with the Cell Model and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble
Prognostic factors in lumbar spinal stenosis surgery. A prospective study of imaging- and patient-related factors in 109 patients who were operated on by decompression
Background and purpose A considerable number of patients who undergo surgery for spinal stenosis have residual symptoms and inferior function and health-related quality of life after surgery. There have been few studies on factors that may predict outcome. We tried to find predictors of outcome in surgery for spinal stenosis using patient- and imaging-related factors. Patients and methods 109 patients in the Swedish Spine Register with central spinal stenosis that were operated on by decompression without fusion were prospectively followed up 1 year after surgery. Clinical outcome scores included the EQ-5D, the Oswestry disability index, self-estimated walking distance, and leg and back pain levels (VAS). Central dural sac area, number of levels with stenosis, and spondylolisthesis were included in the MRI analysis. Multivariable analyses were performed to search for correlation between patient-related and imaging factors and clinical outcome at 1-year follow-up. Results Several factors predicted outcome statistically significantly. Duration of leg pain exceeding 2 years predicted inferior outcome in terms of leg and back pain, function, and HRLQoL. Regular and intermittent preoperative users of analgesics had higher levels of back pain at follow-up than those not using analgesics. Low preoperative function predicted low function and dissatisfaction at follow-up. Low preoperative EQ-5D scores predicted a high degree of leg and back pain. Narrow dural sac area predicted more gains in terms of back pain at follow-up and lower absolute leg pain. Interpretation Multiple factors predict outcome in spinal stenosis surgery, most importantly duration of symptoms and preoperative function. Some of these are modifiable and can be targeted. Our findings can be used in the preoperative patient information and aid the surgeon and the patient in a shared decision making process
A Variational Approach for Minimizing Lennard-Jones Energies
A variational method for computing conformational properties of molecules
with Lennard-Jones potentials for the monomer-monomer interactions is
presented. The approach is tailored to deal with angular degrees of freedom,
{\it rotors}, and consists in the iterative solution of a set of deterministic
equations with annealing in temperature. The singular short-distance behaviour
of the Lennard-Jones potential is adiabatically switched on in order to obtain
stable convergence. As testbeds for the approach two distinct ensembles of
molecules are used, characterized by a roughly dense-packed ore a more
elongated ground state. For the latter, problems are generated from natural
frequencies of occurrence of amino acids and phenomenologically determined
potential parameters; they seem to represent less disorder than was previously
assumed in synthetic protein studies. For the dense-packed problems in
particular, the variational algorithm clearly outperforms a gradient descent
method in terms of minimal energies. Although it cannot compete with a careful
simulating annealing algorithm, the variational approach requires only a tiny
fraction of the computer time. Issues and results when applying the method to
polyelectrolytes at a finite temperature are also briefly discussed.Comment: 14 pages, uuencoded compressed postscript fil
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