2,661 research outputs found
Singularity results for functional equations driven by linear fractional transformations
We consider functional equations driven by linear fractional transformations,
which are special cases of de Rham's functional equations. We consider
Hausdorff dimension of the measure whose distribution function is the solution.
We give a necessary and sufficient condition for singularity. We also show that
they have a relationship with stationary measures.Comment: 14 pages, Title changed, to appear in Journal of Theoretical
Probabilit
Design of Optimum Ducts Using an Efficient 3-D Viscous Computational Flow Analysis
Design of fluid dynamically efficient ducts is addressed through the combination of an optimization analysis with a three-dimensional viscous fluid dynamic analysis code. For efficiency, a parabolic fluid dynamic analysis was used. Since each function evaluation in an optimization analysis is a full three-dimensional viscous flow analysis requiring 200,000 grid points, it is important to use both an efficient fluid dynamic analysis and an efficient optimization technique. Three optimization techniques are evaluated on a series of test functions. The Quasi-Newton (BFGS, eta = .9) technique was selected as the preferred technique. A series of basic duct design problems are performed. On a two-parameter optimization problem, the BFGS technique is demonstrated to require half as many function evaluations as a steepest descent technique
A lattice model of hydrophobic interactions
Hydrogen bonding is modeled in terms of virtual exchange of protons between
water molecules. A simple lattice model is analyzed, using ideas and techniques
from the theory of correlated electrons in metals. Reasonable parameters
reproduce observed magnitudes and temperature dependence of the hydrophobic
interaction between substitutional impurities and water within this lattice.Comment: 7 pages, 3 figures. To appear in Europhysics Letter
Rehabilitation following surgery for lumbar spinal stenosis: a Cochrane review
Study Design
A systematic review of randomised controlled trials (RCTs)
Objective
To determine the effects of active rehabilitation on functional outcome following lumbar spinal stenosis surgery when compared with 'usual postoperative care'.
Summary of background data
Surgery rates for lumbar spinal stenosis have risen, yet outcomes remain suboptimal. Post-operative rehabilitation has been suggested as a tool to improve post-operative function but, to date, there is limited evidence to support its use.
Methods
CENTRAL (The Cochrane Library), the Cochrane Back Review Group Trials Register, MEDLINE, EMBASE, CINAHL and PEDro electronic databases were searched. Randomised controlled trials (RCTs) comparing the effectiveness of active rehabilitation with usual care in adults with lumbar spinal stenosis who had undergone primary spinal decompression surgery were included. Two authors independently selected studies, assessed the risk of bias, and extracted the data in line with the recommendations of the Cochrane Back Review Group. Study results were pooled in a meta-analysis when appropriate using functional status as the primary outcome, with secondary outcomes including measures of leg pain, low back pain, and global improvement/general health. The GRADE approach was used to assess the quality of the evidence.
Results
Our searches yielded 1,726 articles, of which three studies (N = 373 participants) were suitable for inclusion in meta-analysis. All included studies were deemed to have low risk of bias; no study had unacceptably high dropout rates. There was moderate evidence suggesting that active rehabilitation was more effective than usual care in improving both short- and long-term functional status following surgery. Similar findings were noted for secondary outcomes, including short-term improvement in low back pain and long-term improvement in both low back pain and leg pain.
Conclusions
We obtained moderate-quality evidence indicating that postoperative active rehabilitation after decompression surgery for lumbar spinal stenosis is more effective than usual care. Further work is required particularly with respect to the cost effectiveness of such interventions
Organized condensation of worm-like chains
We present results relevant to the equilibrium organization of DNA strands of
arbitrary length interacting with a spherical organizing center, suggestive of
DNA-histone complexation in nucleosomes. We obtain a rich phase diagram in
which a wrapping state is transformed into a complex multi-leafed, rosette
structure as the adhesion energy is reduced. The statistical mechanics of the
"melting" of a rosette can be mapped into an exactly soluble one-dimensional
many-body problem.Comment: 15 pages, 2 figures in a pdf fil
Rods Near Curved Surfaces and in Curved Boxes
We consider an ideal gas of infinitely rigid rods near a perfectly repulsive
wall, and show that the interfacial tension of a surface with rods on one side
is lower when the surface bends towards the rods. Surprisingly we find that
rods on both sides of surfaces also lower the energy when the surface bends. We
compute the partition functions of rods confined to spherical and cylindrical
open shells, and conclude that spherical shells repel rods, whereas cylindrical
shells (for thickness of the shell on the order of the rod-length) attract
them. The role of flexibility is investigated by considering chains composed of
two rigid segments.Comment: 39 pages including figures and tables. 12 eps figures. LaTeX with
REVTe
Role of Multipoles in Counterion-Mediated Interactions between Charged Surfaces: Strong and Weak Coupling
We present general arguments for the importance, or lack thereof, of the
structure in the charge distribution of counterions for counterion-mediated
interactions between bounding symmetrically charged surfaces. We show that on
the mean field or weak coupling level, the charge quadrupole contributes the
lowest order modification to the contact value theorem and thus to the
intersurface electrostatic interactions. The image effects are non-existent on
the mean-field level even with multipoles. On the strong coupling level the
quadrupoles and higher order multipoles contribute additional terms to the
interaction free energy only in the presence of dielectric inhomogeneities.
Without them, the monopole is the only multipole that contributes to the strong
coupling electrostatics. We explore the consequences of these statements in all
their generality.Comment: 12 pages, 3 figure
Screening by symmetry of long-range hydrodynamic interactions of polymers confined in sheets
Hydrodynamic forces may significantly affect the motion of polymers. In
sheet-like cavities, such as the cell's cytoplasm and microfluidic channels,
the hydrodynamic forces are long-range. It is therefore expected that that
hydrodynamic interactions will dominate the motion of polymers in sheets and
will be manifested by Zimm-like scaling. Quite the opposite, we note here that
although the hydrodynamic forces are long-range their overall effect on the
motion of polymers vanishes due to the symmetry of the two-dimensional flow. As
a result, the predicted scaling of experimental observables such as the
diffusion coefficient or the rotational diffusion time is Rouse-like, in accord
with recent experiments. The effective screening validates the use of the
non-interacting blobs picture for polymers confined in a sheet.Comment: http://www.weizmann.ac.il/complex/tlusty/papers/Macromolecules2006.pdf
http://pubs.acs.org/doi/abs/10.1021/ma060251
Charge-Fluctuation-Induced Non-analytic Bending Rigidity
In this Letter, we consider a neutral system of mobile positive and negative
charges confined on the surface of curved films. This may be an appropriate
model for: i) a highly charged membrane whose counterions are confined to a
sheath near its surface; ii) a membrane composed of an equimolar mixture of
anionic and cationic surfactants in aqueous solution. We find that the charge
fluctuations contribute a non-analytic term to the bending rigidity that varies
logarithmically with the radius of curvature. This may lead to spontaneous
vesicle formation, which is indeed observed in similar systems.Comment: Revtex, 9 pages, no figures, submitted to PR
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