2,968 research outputs found
Enhancement by polydispersity of the biaxial nematic phase in a mixture of hard rods and plates
The phase diagram of a polydisperse mixture of uniaxial rod-like and
plate-like hard parallelepipeds is determined for aspect ratios and
15. All particles have equal volume and polydispersity is introduced in a
highly symmetric way. The corresponding binary mixture is known to have a
biaxial phase for , but to be unstable against demixing into two
uniaxial nematics for . We find that the phase diagram for
is qualitatively similar to that of the binary mixture, regardless
the amount of polydispersity, while for a sufficient amount of
polydispersity stabilizes the biaxial phase. This provides some clues for the
design of an experiment in which this long searched biaxial phase could be
observed.Comment: 4 pages, 5 eps figure files, uses RevTeX 4 styl
Angioarchitectural evolution of clival dural arteriovenous fistulas in two patients.
Dural arteriovenous fistulas (dAVFs) may present in a variety of ways, including as carotid-cavernous sinus fistulas. The ophthalmologic sequelae of carotid-cavernous sinus fistulas are known and recognizable, but less commonly seen is the rare clival fistula. Clival dAVFs may have a variety of potential anatomical configurations but are defined by the involvement of the venous plexus just overlying the bony clivus. Here we present two cases of clival dAVFs that most likely evolved from carotid-cavernous sinus fistulas
Interpolating Action for Strings and Membranes - a Study of Symmetries in the Constrained Hamiltonian Approach
A master action for bosonic strings and membranes, interpolating between the
Nambu--Goto and Polyakov formalisms, is discussed. The role of the gauge
symmetries vis-\`{a}-vis reparametrization symmetries of the various actions is
analyzed by a constrained Hamiltonian approach. This analysis reveals the
difference between strings and higher branes, which is essentially tied to a
degree of freedom count. The cosmological term for membranes follows naturally
in this scheme. The conncetion of our aproach with the Arnowitt--Deser--Misner
representation in general relativity is illuminated.Comment: LaTex, 23 pages; discussion on ADM representation included and new
references adde
Nonlocal lattice fermion models on the 2d torus
Abelian fermion models described by the SLAC action are considered on a
finite 2d lattice. It is shown that modification of these models by introducing
additional Pauli - Villars regularization supresses nonlocal effects and
provides agreement with the continuum results in vectorial U(1) models. In the
case of chiral fermions the phase of the determinant differs from the continuum
one.Comment: 16 pages, LaTeX, 5 eps figures, uses epsf.sty, rotate.st
Bending the Elbow During Shoulder Flexion Facilitates Greater Scapular Upward Rotation and a More Favorable Scapular Muscle Activation Pattern
Context: Decreased scapular upward rotation (UR) and diminished activation of the serratus anterior (SA) and lower trapezius (LT) are often observed among patients with subacromial impingement syndrome. Maintaining the elbow fully flexed during shoulder flexion may limit glenohumeral motion due to passive insufficiency of the triceps brachii and therefore facilitate greater scapular UR and increased scapular muscle activation. Objectives: To compare scapular UR, SA, upper trapezius (UT), middle trapezius, and LT activation levels between shoulder flexion with the elbow extended (Flexion-EE) to shoulder flexion with the elbow fully flexed (Flexion-EF). This study hypothesized that Flexion-EF would result in greater scapular UR, greater SA and LT activation, and a lower UT/SA and UT/LT activation ratio compared with Flexion-EE. Design: Cross-sectional study. Setting: A clinical biomechanics laboratory. Participants: Twenty-two healthy individuals. Main Outcome Measures: Scapular UR and electromyography signal of the SA, UT, middle trapezius, and LT, as well as UT/SA and UT/LT activation ratio were measured during Flexion-EE and Flexion-EF. Results: Flexion-EF resulted in greater scapular UR compared with Flexion-EE (P \u3c .001). Flexion-EF resulted in greater SA activation, lower UT activation, and a lower UT/SA activation ratio compared with Flexion-EE (P \u3c .001). Conclusions: Fully flexing the elbow during shoulder flexion leads to increased scapular UR primarily through greater activation of the SA. This exercise may be of value in circumstances involving diminished scapular UR, decreased activation of the SA, and an overly active UT such as among patients with subacromial impingement syndrome
DNA in nanopore-counterion condensation and coion depletion
Molecular dynamics simulations are used to study the equilibrium distribution
of monovalent ions in a nanopore connecting two water reservoirs separated by a
membrane, both for the empty pore and that with a single stranded DNA molecule
inside. In the presence of DNA, the counterions condense on the stretched
macromolecule effectively neutralizing it, and nearly complete depletion of
coions from the pore is observed. The implications of our results for
experiments on DNA translocation through alpha-hemolysin nanopores are
discussed.Comment: 8 pages, 2 figure
Stretching Instability of Helical Spring
We show that when a gradually increasing tensile force is applied to the ends
of a helical spring with sufficiently large ratios of radius to pitch and twist
to bending rigidity, the end-to-end distance undergoes a sequence of
discontinuous stretching transitions. Subsequent decrease of the force leads to
step-like contraction and hysteresis is observed. For finite helices, the
number of these transitions increases with the number of helical turns but only
one stretching and one contraction instability survive in the limit of an
infinite helix. We calculate the critical line that separates the region of
parameters in which the deformation is continuous from that in which stretching
instabilities occur, and propose experimental tests of our predictions.Comment: 5 pages, 4 figure
Dual Projection and Selfduality in Three Dimensions
We discuss the notion of duality and selfduality in the context of the dual
projection operation that creates an internal space of potentials. Contrary to
the prevailing algebraic or group theoretical methods, this technique is
applicable to both even and odd dimensions. The role of parity in the kernel of
the Gauss law to determine the dimensional dependence is clarified. We derive
the appropriate invariant actions, discuss the symmetry groups and their proper
generators. In particular, the novel concept of duality symmetry and
selfduality in Maxwell theory in (2+1) dimensions is analysed in details. The
corresponding action is a 3D version of the familiar duality symmetric
electromagnetic theory in 4D. Finally, the duality symmetric actions in the
different dimensions constructed here manifest both the SO(2) and
symmetries, contrary to conventional results.Comment: 20 pages, late
Probabilistic Algorithmic Knowledge
The framework of algorithmic knowledge assumes that agents use deterministic
knowledge algorithms to compute the facts they explicitly know. We extend the
framework to allow for randomized knowledge algorithms. We then characterize
the information provided by a randomized knowledge algorithm when its answers
have some probability of being incorrect. We formalize this information in
terms of evidence; a randomized knowledge algorithm returning ``Yes'' to a
query about a fact \phi provides evidence for \phi being true. Finally, we
discuss the extent to which this evidence can be used as a basis for decisions.Comment: 26 pages. A preliminary version appeared in Proc. 9th Conference on
Theoretical Aspects of Rationality and Knowledge (TARK'03
Magnification relations in gravitational lensing via multidimensional residue integrals
We investigate the so-called magnification relations of gravitational lensing
models. We show that multidimensional residue integrals provide a simple
explanation for the existence of these relations, and an effective method of
computation. We illustrate the method with several examples, thereby deriving
new magnification relations for galaxy lens models and microlensing (point mass
lensing).Comment: 16 pages, uses revtex4, submitted to Journal of Mathematical Physic
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