20,272 research outputs found
On toric geometry, Spin(7) manifolds, and type II superstring compactifications
We consider type II superstring compactifications on the singular Spin(7)
manifold constructed as a cone on SU(3)/U(1). Based on a toric realization of
the projective space CP^2, we discuss how the manifold can be viewed as three
intersecting Calabi-Yau conifolds. The geometric transition of the manifold is
then addressed in this setting. The construction is readily extended to higher
dimensions where we speculate on possible higher-dimensional geometric
transitions. Armed with the toric description of the Spin(7) manifold, we
discuss a brane/flux duality in both type II superstring theories compactified
on this manifold.Comment: 14 pages, v2: version to be publishe
Higher su(N) tensor products
We extend our recent results on ordinary su(N) tensor product multiplicities
to higher su(N) tensor products. Particular emphasis is put on four-point
couplings where the tensor product of four highest weight modules is
considered. The number of times the singlet occurs in the decomposition is the
associated multiplicity. In this framework, ordinary tensor products correspond
to three-point couplings. As in that case, the four-point multiplicity may be
expressed explicitly as a multiple sum measuring the discretised volume of a
convex polytope. This description extends to higher-point couplings as well. We
also address the problem of determining when a higher-point coupling exists,
i.e., when the associated multiplicity is non-vanishing. The solution is a set
of inequalities in the Dynkin labels.Comment: 17 pages, LaTe
Effective Potential Theory: A Practical Way to Extend Plasma Transport Theory to Strong Coupling
The effective potential theory is a physically motivated method for extending
traditional plasma transport theories to stronger coupling. It is practical in
the sense that it is easily incorporated within the framework of the
Chapman-Enskog or Grad methods that are commonly applied in plasma physics and
it is computationally efficient to evaluate. The extension is to treat binary
scatterers as interacting through the potential of mean force, rather than the
bare Coulomb or Debye-screened Coulomb potential. This allows for aspects of
many-body correlations to be included in the transport coefficients. Recent
work has shown that this method accurately extends plasma theory to orders of
magnitude stronger coupling when applied to the classical one-component plasma
model. The present work shows that similar accuracy is realized for the Yukawa
one-component plasma model and it provides a comparison with other approaches.Comment: 6 pages, 3 figures, Proceedings of the Strongly Coupled Coulomb
Systems conference 201
New State Record and Notable Range Extension for \u3ci\u3eLibellula Semifasciata\u3c/i\u3e (Odonata: Libellulidae)
The painted skimmer, Libellula semifasciata Burmeister (Odonata: Libellulidae), is an eastern species of dragonfly that has never been documented in Iowa. In this note we report two observations and the collection of a voucher for this species in southeast Iowa in the last three years. Based on other records of this species, including those from neighboring states and more northerly latitudes, we propose that these observations are evidence of a range extension
Adaptive, cautious, predictive control with Gaussian process priors
Nonparametric Gaussian Process models, a Bayesian statistics approach, are used to implement a nonlinear adaptive control law. Predictions, including propagation of the state uncertainty are made over a k-step horizon. The expected value of a quadratic cost function is minimised, over this prediction horizon, without ignoring the variance of the model predictions. The general method and its main features are illustrated on a simulation example
A Tight Lower Bound to the Outage Probability of Discrete-Input Block-Fading Channels
In this correspondence, we propose a tight lower bound to the outage
probability of discrete-input Nakagami-m block-fading channels. The approach
permits an efficient method for numerical evaluation of the bound, providing an
additional tool for system design. The optimal rate-diversity trade-off for the
Nakagami-m block-fading channel is also derived and a tight upper bound is
obtained for the optimal coding gain constant.Comment: 22 pages, 4 figures. This work has been accepted for IEEE
Transactions on Information Theory and has been presented in part at the 2007
IEEE International Symposium on Information Theory, Nice, France, June 200
Transfer of BECs through discrete breathers in an optical lattice
We study the stability of a stationary discrete breather (DB) on a nonlinear
trimer in the framework of the discrete nonlinear Schr\"odinger equation
(DNLS). In previous theoretical investigations of the dynamics of Bose-Einstein
condensates in leaking optical lattices, collisions between a DB and a lattice
excitation, e.g. a moving breather (MB) or phonon, were studied. These
collisions lead to the transmission of a fraction of the incident (atomic) norm
of the MB through the DB, while the DB can be shifted in the direction of the
incident lattice excitation. Here we show that there exists a total energy
threshold of the trimer, above which the lattice excitation can trigger the
destabilization of the DB and that this is the mechanism leading to the
movement of the DB. Furthermore, we give an analytic estimate of upper bound to
the norm that is transmitted through the DB. Our analysis explains the results
of the earlier numerical studies and may help to clarify functional operations
with BECs in optical lattices such as blocking and filtering coherent (atomic)
beams.Comment: 8 pages, 5 figure
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