18,686 research outputs found
Amenability of Groupoids Arising from Partial Semigroup Actions and Topological Higher Rank Graphs
We consider the amenability of groupoids equipped with a group valued
cocycle with amenable kernel . We prove a general result
which implies, in particular, that is amenable whenever is amenable and
if there is countable set such that for all . We show that our result is applicable to groupoids arising from
partial semigroup actions. We explore these actions in detail and show that
these groupoids include those arising from directed graphs, higher rank graphs
and even topological higher rank graphs. We believe our methods yield a nice
alternative groupoid approach to these important constructions.Comment: Revised as suggested by a very helpful referee. In particular, a gap
in the proof of Theorem 5.13 has been repaired resulting in a much improved
version (with fewer hypotheses
Fractional Boundaries for Fluid Spheres
A single Israel layer can be created when two metrics adjoin with no
continuous metric derivative across the boundary. The properties of the layer
depend only on the two metrics it separates. By using a fractional derivative
match, a family of Israel layers can be created between the same two metrics.
The family is indexed by the order of the fractional derivative. The method is
applied to Tolman IV and V interiors and a Schwarzschild vacuum exterior. The
method creates new ranges of modeling parameters for fluid spheres. A thin
shell analysis clarifies pressure/tension in the family of boundary layers.Comment: to appear in J. Math. Phy
Jensen-Feynman approach to the statistics of interacting electrons
Faussurier et al. [Phys. Rev. E 65, 016403 (2001)] proposed to use a
variational principle relying on Jensen-Feynman (or Gibbs-Bogoliubov)
inequality in order to optimize the accounting for two-particle interactions in
the calculation of canonical partition functions. It consists in a
decomposition into a reference electron system and a first-order correction.
The procedure appears to be very efficient in order to evaluate the free energy
and the orbital populations. In this work, we present numerical applications of
the method and propose to extend it using a reference energy which includes the
interaction between two electrons inside a given orbital. This is possible
thanks to our efficient recursion relation for the calculation of partition
functions. We also show that a linear reference energy, however, is usually
sufficient to achieve a good precision and that the most promising way to
improve the approach of Faussurier et al. is to apply Jensen's inequality to a
more convenient convex function.Comment: submitted to Physical Review
Dynamics of the formation of a hydrogel by a pathogenic amyloid peptide: islet amyloid polypeptide
Many chronic degenerative diseases result from aggregation of misfolded polypeptides to form amyloids. Many amyloidogenic polypeptides are surfactants and their assembly can be catalysed by hydrophobic-hydrophilic interfaces (an air-water interface in-vitro or membranes in-vivo). We recently demonstrated the specificity of surface-induced amyloidogenesis but the mechanisms of amyloidogenesis and more specifically of adsorption at hydrophobic-hydrophilic interfaces remain poorly understood. Thus, it is critical to determine how amyloidogenic polypeptides behave at interfaces. Here we used surface tensiometry, rheology and electron microscopy to demonstrate the complex dynamics of gelation by full-length human islet amyloid polypeptide (involved in type II diabetes) both in the bulk solution and at hydrophobic-hydrophilic interfaces (air-water interface and phospholipids). We show that the hydrogel consists of a 3D supramolecular network of fibrils. We also assessed the role of solvation and dissected the evolution over time of the assembly processes. Amyloid gelation could have important pathological consequences for membrane integrity and cellular functions
Bose-Einstein transition temperature in a dilute repulsive gas
We discuss certain specific features of the calculation of the critical
temperature of a dilute repulsive Bose gas. Interactions modify the critical
temperature in two different ways. First, for gases in traps, temperature
shifts are introduced by a change of the density profile, arising itself from a
modification of the equation of state of the gas (reduced compressibility);
these shifts can be calculated simply within mean field theory. Second, even in
the absence of a trapping potential (homogeneous gas in a box), temperature
shifts are introduced by the interactions; they arise from the correlations
introduced in the gas, and thus lie inherently beyond mean field theory - in
fact, their evaluation requires more elaborate, non-perturbative, calculations.
One illustration of this non-perturbative character is provided by the solution
of self-consistent equations, which relate together non-linearly the various
energy shifts of the single particle levels k. These equations predict that
repulsive interactions shift the critical temperature (at constant density) by
an amount which is positive, and simply proportional to the scattering length
a; nevertheless, the numerical coefficient is difficult to compute. Physically,
the increase of the temperature can be interpreted in terms of the reduced
density fluctuations introduced by the repulsive interactions, which facilitate
the propagation of large exchange cycles across the sample.Comment: two minor corrections, two refs adde
Nuclear star formation on 100 parsec scales: 10" resolution radio continuum, HI and CO observations
A program of radio line and continuum studies of star formation in nearby spiral galaxies is reported. The objective is a search for hot gas and peculiar dynamics in spiral nuclei with 10" to 30" angular resolution. Vigorous star formation is found to be a common phenomenon in the inner kpc of spirals. Arcsecond resolution observations of radio continuum emission at 6 and 2 cm were used to separate the thermal and nonthermal radio components. It was found that thermal and nonthermal emission are well mixed even on sizescales of 10 pc. To understand the reason for the increased level of star formation activity in spiral nuclei, HI and CO emission in these galaxies is studied. The CO transition was detected in M51, M82, NGC 253, NGC 6946 and IC 342 with T sub a approx. 0.5 to 2.0 K, at 20" angular resolution. The dynamics and spatial distribution of nuclear gas are being studied using VLA HI maps with 30" synthesized beams. Evidence for noncircular motions in HI was found in the nucleus of IC 342
Two-photon decay of pseudoscalar quarkonia
We report on our recent evaluation of the two-photon width of the
pseudoscalar quarkonia, eta_c(nS) and eta_b(nS) in an approach based on
Heavy-Quark Spin Symmetry (HQSS). To what concerns the 1S state eta_c, our
parameter-free computation agrees with experiments, as well as most of other
theoretical works. On the other hand, our computation for the 2S-state looks 2S
like a confirmation that there may exist an anomaly related to the decay of
eta_c(2S), especially in the light of the new preliminary result of the Belle
collaboration. We also point out that the essentially model-independent ratio
of eta_b two-photon width to the Upsilon leptonic width and the eta_b
two-photon width could be used to extract the strong coupling constant alpha_s.Comment: Presented by T.N. Pham at the Joint Meeting
Heidelberg-Liege-Paris-Wroclaw (HLPW08), Spa, Belgium, 6-8 March 2008, 9
pages, 2 figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty
(included
Site occupation constraints in mean-field approaches of quantum spin systems at finite temperature
We study the effect of site occupation on the description of quantum spin
systems at finite temperature and mean-field level. We impose each lattice site
to be occupied by a single electron. This is realized by means of a specific
prescription. The outcome of the prescription is compared to the result
obtained by means of a projection procedure which fixes the site occupation to
one particle per site on an average. The comparison is performed for different
representations of the Hamiltonian in Fock space leading to different types of
mean-field solutions. The behaviour of order parameters is analyzed for each
choice of the mean-field and constraint which fixes the occupation rate at each
site. Sizable quantitative differences between the outcomes obtained with the
two different constraints are observed.Comment: 18 pages, 2 figure
A System of Interaction and Structure II: The Need for Deep Inference
This paper studies properties of the logic BV, which is an extension of
multiplicative linear logic (MLL) with a self-dual non-commutative operator. BV
is presented in the calculus of structures, a proof theoretic formalism that
supports deep inference, in which inference rules can be applied anywhere
inside logical expressions. The use of deep inference results in a simple
logical system for MLL extended with the self-dual non-commutative operator,
which has been to date not known to be expressible in sequent calculus. In this
paper, deep inference is shown to be crucial for the logic BV, that is, any
restriction on the ``depth'' of the inference rules of BV would result in a
strictly less expressive logical system
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