11,340 research outputs found
Directly Indecomposables in Semidegenerate Varieties of Connected po-Groupoids
We study varieties with a term-definable poset structure, "po-groupoids". It
is known that connected posets have the "strict refinement property" (SRP). In
[arXiv:0808.1860v1 [math.LO]] it is proved that semidegenerate varieties with
the SRP have definable factor congruences and if the similarity type is finite,
directly indecomposables are axiomatizable by a set of first-order sentences.
We obtain such a set for semidegenerate varieties of connected po-groupoids and
show its quantifier complexity is bounded in general
Model Independent Bound on the Unitarity Triangle from CP Violation in B-> pi+ pi- and B-> psi K_S
We derive model independent lower bounds on the CKM parameters (1-rhobar) and
etabar as functions of the mixing-induced CP asymmetry S in B-> pi+ pi- and
sin(2 beta) from B->psi K_S. The bounds do not depend on specific results of
theoretical calculations for the penguin contribution to B-> pi+ pi-. They
require only the very conservative condition that a hadronic phase, which
vanishes in the heavy-quark limit, does not exceed 90 degrees in magnitude. The
bounds are effective if -sin(2 beta) < S < 1. Dynamical calculations indicate
that the limits on rhobar and etabar are close to their actual values.Comment: 5 pages, 2 figure
Swinging and Tumbling of Fluid Vesicles in Shear Flow
The dynamics of fluid vesicles in simple shear flow is studied using
mesoscale simulations of dynamically-triangulated surfaces, as well as a
theoretical approach based on two variables, a shape parameter and the
inclination angle, which has no adjustable parameters. We show that between the
well-known tank-treading and tumbling states, a new ``swinging'' state can
appear. We predict the dynamic phase diagram as a function of the shear rate,
the viscosities of the membrane and the internal fluid, and the reduced vesicle
volume. Our results agree well with recent experiments.Comment: 4 pages, 4 figure
Ion Pair Potentials-of-Mean-Force in Water
Recent molecular simulation and integral equation results alkali-halide ion
pair potentials-of-mean-force in water are discussed. Dielectric model
calculations are implemented to check that these models produce that
characteristic structure of contact and solvent-separated minima for oppositely
charged ions in water under physiological thermodynamic conditions. Comparison
of the dielectric model results with the most current molecular level
information indicates that the dielectric model does not, however, provide an
accurate description of these potentials-of-mean-force. We note that linear
dielectric models correspond to modelistic implementations of second-order
thermodynamic perturbation theory for the excess chemical potential of a
distinguished solute molecule. Therefore, the molecular theory corresponding to
the dielectric models is second-order thermodynamic perturbation theory for
that excess chemical potential. The second-order, or fluctuation, term raises a
technical computational issue of treatment of long-ranged interactions similar
to the one which arises in calculation of the dielectric constant of the
solvent. It is contended that the most important step for further development
of dielectric models would be a separate assessment of the first-order
perturbative term (equivalently the {\it potential at zero charge} ) which
vanishes in the dielectric models but is generally nonzero. Parameterization of
radii and molecular volumes should then be based of the second-order
perturbative term alone. Illustrative initial calculations are presented and
discussed.Comment: 37 pages and 8 figures. LA-UR-93-420
Ion Sizes and Finite-Size Corrections for Ionic-Solvation Free Energies
Free energies of ionic solvation calculated from computer simulations exhibit
a strong system size dependence. We perform a finite-size analysis based on a
dielectric-continuum model with periodic boundary conditions. That analysis
results in an estimate of the Born ion size. Remarkably, the finite-size
correction applies to systems with only eight water molecules hydrating a
sodium ion and results in an estimate of the Born radius of sodium that agrees
with the experimental value.Comment: 2 EPS figure
- …