84,813 research outputs found
Performance of a worm algorithm in theory at finite quartic coupling
Worm algorithms have been very successful with the simulation of sigma models
with fixed length spins which result from scalar field theories in the limit of
infinite quartic coupling lambda. Here we investigate closer their algorithmic
efficiency at finite and even vanishing lambda for the one component model in
dimensions D = 2, 3, 4.Comment: 10 pages, 2 Fig
Simulating the All-Order Strong Coupling Expansion III: O(N) sigma/loop models
We reformulate the O(N) sigma model as a loop model whose configurations are
the all-order strong coupling graphs of the original model. The loop
configurations are represented by a pointer list in the computer and a Monte
Carlo update scheme is proposed. Sample simulations are reported and the method
turns out to be similarly efficient as the reflection cluster method, but it
has greater potential for systematic generalization to other lattice field
theories. A variant action suggested by the method is also simulated and leads
to a rather extreme demonstration of the concept of universality of the scaling
or continuum limit.Comment: 27 pages, 6 figures, 5 tables. The combinatoric derivation of the
(unchanged) algorithm is corrected. Independently, all data in sect.5 are
revised due to a software error, with unchanged conclusions on universality
Simulating the All-Order Strong Coupling Expansion I: Ising Model Demo
We investigate in some detail an alternative simulation strategy for lattice
field theory based on the so-called worm algorithm introduced by Prokof'ev and
Svistunov in 2001. It amounts to stochastically simulating the strong coupling
expansion rather than the usual configuration sum. A detailed error analysis
and an important generalization of the method are exemplified here in the
simple Ising model. It allows for estimates of the two point function where in
spite of exponential decay the signal to noise ratio does not degrade at large
separation. Critical slowing down is practically absent. In the outlook some
thoughts on the general applicability of the method are offered.Comment: 15 pages, 2 figures, refs. added, small language changes, to app. in
Nucl. Phys. B[FS
Towards an Intelligent Database System Founded on the SP Theory of Computing and Cognition
The SP theory of computing and cognition, described in previous publications,
is an attractive model for intelligent databases because it provides a simple
but versatile format for different kinds of knowledge, it has capabilities in
artificial intelligence, and it can also function like established database
models when that is required.
This paper describes how the SP model can emulate other models used in
database applications and compares the SP model with those other models. The
artificial intelligence capabilities of the SP model are reviewed and its
relationship with other artificial intelligence systems is described. Also
considered are ways in which current prototypes may be translated into an
'industrial strength' working system
An Improved Estimator for the Correlation Function of 2D Nonlinear Sigma Models
I present a new improved estimator for the correlation function of 2D
nonlinear sigma models. Numerical tests for the 2D XY model and the 2D
O(3)-invariant vector model were performed. For small physical volume, i.e. a
lattice size small compared to the to the bulk correlation length, a reduction
of the statistical error of the finite system correlation length by a factor of
up to 30 compared to the cluster-improved estimator was observed. This
improvement allows for a very accurate determination of the running coupling
proposed by M. L"uscher et al. for 2D O(N)-invariant vector models.Comment: 20 pages, LaTeX + 2 ps figures, CERN-TH.7375/9
Search for pulsations in the LMXB EXO 0748-676
We present here results from our search for X-ray pulsations of the neutron
star in the low mass X-ray binary EXO 0748-676 at a frequency near a
burst-oscillation frequency of 44.7 Hz. Using the observations made with the
Proportional Counter Array on board the Rossi X-ray Timing Explorer we did not
find any pulsations in the frequency band of 44.4 Hz to 45.0 Hz and obtained a
3\sigma upper limit of 0.47 % on the pulsed fraction for any possible
underlying pulsation in this frequency band.Comment: Accepted for publication in "Research in Astronomy and Astrophysics",
7 pages, 3 figure
Scaling topological charge in the CP^3 spin model
The CP^3 spin model is simulated at large correlation lengths in two
dimensions. An overrelaxation algorithm is employed which yields reduced
critical slowing down with dynamical exponents z around unity. We compare our
results with recent multigrid data on the massgap m and the spin susceptibility
and confirm the absence of asymptotic scaling. As a new result we find scaling
for the universal topological susceptibility with values extrapolating to chi_t
/ m^2 = 0.156(2) in the continuum limit.Comment: 11 pages, 3 ps-figure
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