12,593 research outputs found
Simplicial Quantum Gravity on a Randomly Triangulated Sphere
We study 2D quantum gravity on spherical topologies employing the Regge
calculus approach with the dl/l measure. Instead of the normally used fixed
non-regular triangulation we study random triangulations which are generated by
the standard Voronoi-Delaunay procedure. For each system size we average the
results over four different realizations of the random lattices. We compare
both types of triangulations quantitatively and investigate how the difference
in the expectation value of the squared curvature, , for fixed and random
triangulations depends on the lattice size and the surface area A. We try to
measure the string susceptibility exponents through finite-size scaling
analyses of the expectation value of an added -interaction term, using two
conceptually quite different procedures. The approach, where an ultraviolet
cut-off is held fixed in the scaling limit, is found to be plagued with
inconsistencies, as has already previously been pointed out by us. In a
conceptually different approach, where the area A is held fixed, these problems
are not present. We find the string susceptibility exponent in
rough agreement with theoretical predictions for the sphere, whereas the
estimate for appears to be too negative. However, our results
are hampered by the presence of severe finite-size corrections to scaling,
which lead to systematic uncertainties well above our statistical errors. We
feel that the present methods of estimating the string susceptibilities by
finite-size scaling studies are not accurate enough to serve as testing grounds
to decide about a success or failure of quantum Regge calculus.Comment: LaTex, 29 pages, including 9 figure
End-effects of strongly charged polyelectrolytes - a molecular dynamics study
We investigate end-effects in the ion distribution around strongly charged,
flexible polyelectrolytes with a quenched charge distribution by molecular
dynamics simulations of dilute polyelectrolyte solutions. We take the
counterions explicitly into account and calculate the full Coulomb interaction
via an Ewald summation method. We find that the free counterions of the
solution are distributed in such a way that a fraction of the chain charges is
effectively neutralized. This in turn leads to an effective charge distribution
which is similar to those found for weakly charged titrating polyelectrolytes
that have an annealed charge distribution. The delicate interplay between the
electrostatic interactions, the chain conformation and the counterion
distribution is studied in detail as a function of different system parameters
such as the chain length Nm, the charge fraction f, the charged particle
density rho, the ionic strength and the solvent quality. Comparisons are made
with predictions from a scaling theory.Comment: 20 pages, 10 figures. J. Chem. Phys, to appear June 200
Illinois Waterfowl Harvest, Hunter Activity, and Attitudes Toward September Canada Goose Season, Canada Goose Harvest Monitoring System, and Dates for Hunting Seasons, 1997-98
A study of human performance in a rotating environment
Consideration is given to the lack of sufficient data relative to the response of man to the attendant oculovestibular stimulations induced by multi-directional movement of an individual within the rotating environment to provide the required design criteria. This was done to determine the overall impact of artificial gravity simulations on potential design configurations and crew operational procedures. Gross locomotion and fine motor performance were evaluated. Results indicate that crew orientation, rotational rates, vehicle design configurations, and operational procedures may be used to reduce the severity of the adverse effects of the Coriolis and cross-coupled angular accelerations acting on masses moving within a rotating environment. Results further indicate that crew selection, motivation, and short-term exposures to the rotating environment may be important considerations for future crew indoctrination and training programs
Continuous and discrete Clebsch variational principles
The Clebsch method provides a unifying approach for deriving variational
principles for continuous and discrete dynamical systems where elements of a
vector space are used to control dynamics on the cotangent bundle of a Lie
group \emph{via} a velocity map. This paper proves a reduction theorem which
states that the canonical variables on the Lie group can be eliminated, if and
only if the velocity map is a Lie algebra action, thereby producing the
Euler-Poincar\'e (EP) equation for the vector space variables. In this case,
the map from the canonical variables on the Lie group to the vector space is
the standard momentum map defined using the diamond operator. We apply the
Clebsch method in examples of the rotating rigid body and the incompressible
Euler equations. Along the way, we explain how singular solutions of the EP
equation for the diffeomorphism group (EPDiff) arise as momentum maps in the
Clebsch approach. In the case of finite dimensional Lie groups, the Clebsch
variational principle is discretised to produce a variational integrator for
the dynamical system. We obtain a discrete map from which the variables on the
cotangent bundle of a Lie group may be eliminated to produce a discrete EP
equation for elements of the vector space. We give an integrator for the
rotating rigid body as an example. We also briefly discuss how to discretise
infinite-dimensional Clebsch systems, so as to produce conservative numerical
methods for fluid dynamics
The Hamiltonian structure and Euler-Poincar\'{e} formulation of the Vlasov-Maxwell and gyrokinetic systems
We present a new variational principle for the gyrokinetic system, similar to
the Maxwell-Vlasov action presented in Ref. 1. The variational principle is in
the Eulerian frame and based on constrained variations of the phase space fluid
velocity and particle distribution function. Using a Legendre transform, we
explicitly derive the field theoretic Hamiltonian structure of the system. This
is carried out with a modified Dirac theory of constraints, which is used to
construct meaningful brackets from those obtained directly from
Euler-Poincar\'{e} theory. Possible applications of these formulations include
continuum geometric integration techniques, large-eddy simulation models and
Casimir type stability methods.
[1] H. Cendra et. al., Journal of Mathematical Physics 39, 3138 (1998)Comment: 36 pages, 1 figur
Quasi-conservation laws for compressible 3D Navier-Stokes flow
We formulate the quasi-Lagrangian fluid transport dynamics of mass density
and the projection q=\bom\cdot\nabla\rho of the vorticity \bom onto
the density gradient, as determined by the 3D compressible Navier-Stokes
equations for an ideal gas, although the results apply for an arbitrary
equation of state. It turns out that the quasi-Lagrangian transport of
cannot cross a level set of . That is, in this formulation, level sets of
(isopychnals) are impermeable to the transport of the projection .Comment: 2 page note, to appear in Phys Rev
Spatiotemporal patterns and agro-ecological risk factors for cutaneous and renal glomerular vasculopathy (Alabama Rot) in dogs in the UK
Seasonal outbreaks of cutaneous and renal glomerular vasculopathy (CRGV) have been reported annually in UK dogs since 2012, yet the aetiology of the disease remains unknown. The objectives of this study were to explore whether any breeds had an increased or decreased risk of being diagnosed with CRGV, and to report on age and sex distributions of CRGV cases occurring in the UK. Multivariable logistic regression was used to compare 101 dogs diagnosed with CRGV between November 2012 and May 2017 with a denominator population of 446,453 dogs from the VetCompass database. Two Kennel Club breed groups—hounds (odds ratio (OR) 10.68) and gun dogs (OR 9.69)—had the highest risk of being diagnosed with CRGV compared with terriers, while toy dogs were absent from among CRGV cases. Females were more likely to be diagnosed with CRGV (OR 1.51) as were neutered dogs (OR 3.36). As well as helping veterinarians develop an index of suspicion for the disease, better understanding of the signalment risk factors may assist in the development of causal models for CRGV and help identify the aetiology of the disease
Fast Dynamic Graph Algorithms for Parameterized Problems
Fully dynamic graph is a data structure that (1) supports edge insertions and
deletions and (2) answers problem specific queries. The time complexity of (1)
and (2) are referred to as the update time and the query time respectively.
There are many researches on dynamic graphs whose update time and query time
are , that is, sublinear in the graph size. However, almost all such
researches are for problems in P. In this paper, we investigate dynamic graphs
for NP-hard problems exploiting the notion of fixed parameter tractability
(FPT).
We give dynamic graphs for Vertex Cover and Cluster Vertex Deletion
parameterized by the solution size . These dynamic graphs achieve almost the
best possible update time and the query time
, where is the time complexity of any static
graph algorithm for the problems. We obtain these results by dynamically
maintaining an approximate solution which can be used to construct a small
problem kernel. Exploiting the dynamic graph for Cluster Vertex Deletion, as a
corollary, we obtain a quasilinear-time (polynomial) kernelization algorithm
for Cluster Vertex Deletion. Until now, only quadratic time kernelization
algorithms are known for this problem.
We also give a dynamic graph for Chromatic Number parameterized by the
solution size of Cluster Vertex Deletion, and a dynamic graph for
bounded-degree Feedback Vertex Set parameterized by the solution size. Assuming
the parameter is a constant, each dynamic graph can be updated in
time and can compute a solution in time. These results are obtained by
another approach.Comment: SWAT 2014 to appea
-Strands
A -strand is a map for a Lie
group that follows from Hamilton's principle for a certain class of
-invariant Lagrangians. The SO(3)-strand is the -strand version of the
rigid body equation and it may be regarded physically as a continuous spin
chain. Here, -strand dynamics for ellipsoidal rotations is derived as
an Euler-Poincar\'e system for a certain class of variations and recast as a
Lie-Poisson system for coadjoint flow with the same Hamiltonian structure as
for a perfect complex fluid. For a special Hamiltonian, the -strand is
mapped into a completely integrable generalization of the classical chiral
model for the SO(3)-strand. Analogous results are obtained for the
-strand. The -strand is the -strand version of the
Bloch-Iserles ordinary differential equation, whose solutions exhibit dynamical
sorting. Numerical solutions show nonlinear interactions of coherent wave-like
solutions in both cases. -strand equations on the
diffeomorphism group are also introduced and shown
to admit solutions with singular support (e.g., peakons).Comment: 35 pages, 5 figures, 3rd version. To appear in J Nonlin Sc
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