113 research outputs found
Feasibility of nominations in stationary gas networks with random load
The paper considers the computation of the probability of feasible load constellations in a stationary gas network with uncertain demand. More precisely, a network with a single entry and several exits with uncertain loads is studied. Feasibility of a load constellation is understood in the sense of an existing flow meeting these loads along with given pressure bounds in the pipes. In a first step, feasibility of deterministic exit loads is characterized algebraically and these general conditions are specified to networks involving at most one cycle. This prerequisite is essential for determining probabilities in a stochastic setting when exit loads are assumed to follow some (joint) Gaussian distribution when modeling uncertain customer demand. The key of our approach is the application of the spheric-radial decomposition of Gaussian random vectors coupled with Quasi Monte-Carlo sampling. This approach requires an efficient algorithmic treatment of the mentioned algebraic relations moreover depending on a scalar parameter. Numerical results are illustrated for different network examples and demonstrate a clear superiority in terms of precision over simple generic Monte-Carlo sampling. They lead to fairly accurate probability values even for moderate sample size
S-Duality for Linearized Gravity
We develope the analogue of S-duality for linearized gravity in
(3+1)-dimensions. Our basic idea is to consider the self-dual (anti-self-dual)
curvature tensor for linearized gravity in the context of the
Macdowell-Mansouri formalism. We find that the strong-weak coupling duality for
linearized gravity is an exact symmetry and implies small-large duality for the
cosmological constant.Comment: 18 pages, Latex, to be published in Phys. Lett.
Yang--Mills Configurations from 3D Riemann--Cartan Geometry
Recently, the {\it spacelike} part of the Yang--Mills equations has
been identified with geometrical objects of a three--dimensional space of
constant Riemann--Cartan curvature. We give a concise derivation of this
Ashtekar type (``inverse Kaluza--Klein") {\it mapping} by employing a
--decomposition of {\it Clifford algebra}--valued torsion and curvature
two--forms. In the subcase of a mapping to purely axial 3D torsion, the
corresponding Lagrangian consists of the translational and Lorentz {\it
Chern--Simons term} plus cosmological term and is therefore of purely
topological origin.Comment: 14 pages, preprint Cologne-thp-1994-h1
Yang-Mills Interactions and Gravity in Terms of Clifford Algebra
A model of Yang-Mills interactions and gravity in terms of the Clifford
algebra Cl(0,6) is presented. The gravity and Yang-Mills actions are formulated
as different order terms in a generalized action. The feebleness of gravity as
well as the smallness of the cosmological constant and theta terms are
discussed at the classical level. The invariance groups, including the de
Sitter and the Pati-Salam SU(4) subgroups, consist of gauge transformations
from either side of an algebraic spinor. Upon symmetry breaking via the Higgs
fields, the remaining symmetries are the Lorentz SO(1,3), color SU(3),
electromagnetic U(1)_EM, and an additional U(1). The first generation leptons
and quarks are identified with even and odd parts of spinor idempotent
projections. There are still several shortcomings with the current model.
Further research is needed to fully recover the standard model results.Comment: 20 pages, to appear in Advances in Applied Clifford Algebra
Pursuing Gravitational S-Duality
Recently a strong-weak coupling duality in non-abelian non-supersymmetric
theories in four dimensions has been found. An analogous procedure is reviewed,
which allows to find the `dual action' to the gauge theory of dynamical gravity
constructed by the MacDowell-Mansouri model plus the superposition of a
term.Comment: Invited paper to appear in the special issue of the `Journal of
Chaos, Solitons and Fractals' on: "Superstrings, M,F,S,... Theory" (M.S. El
Naschie and C. Castro, editors), 19 pages, LaTeX file, no figure
Gravitational Duality in MacDowell-Mansouri Gauge Theory
Strong-weak duality invariance can only be defined for particular sectors of
supersymmetric Yang-Mills theories. Nevertheless, for full non-Abelian
non-supersymmetric theories, dual theories with inverted couplings, have been
found. We show that an analogous procedure allows to find the dual action to
the gauge theory of gravity constructed by the MacDowell-Mansouri model plus
the superposition of a term.Comment: 9 pages, LaTeX, no figure
Self-dual gravity and self-dual Yang-Mills in the context of Macdowell-Mansouri formalism
In this work we propose an action which unifies self-dual gravity and
self-dual Yang-Mills in the context of the Macdowell-Mansouri formalism. We
claim that such an action may be used to find the S-dual action for both
self-dual gravity and self-dual Yang-Mills.Comment: 8 pages, Revtex, no figures, submitted to Phys. Rev.
Avoiding degenerate coframes in an affine gauge approach to quantum gravity
In quantum models of gravity, it is surmized that configurations with
degenerate coframes could occur during topology change of the underlying
spacetime structure. However, the coframe is not the true Yang--Mills type
gauge field of the translations, since it lacks the inhomogeneous gradient term
in the gauge transformations. By explicitly restoring this ``hidden" piece
within the framework of the affine gauge approach to gravity, one can avoid the
metric or coframe degeneracy which would otherwise interfere with the
integrations within the path integral. This is an important advantage for
quantization.Comment: 14 pages, Preprint Cologne-thp-1993-H
Remarks on 2+1 Self-dual Chern-Simons Gravity
We study 2+1 Chern-Simons gravity at the classical action level. In
particular we rederive the linear combinations of the ``standard'' and
``exotic'' Einstein actions, from the (anti) self-duality of the ``internal''
Lorentzian indices. The relation to a genuine four-dimensional (anti)self-dual
topological theory greatly facilitates the analysis and its relation to
hyperbolic three-dimensional geometry. Finally a non-abelian vector field
``dual'' action is also obtained.Comment: 16+1 pages, LaTeX file, no figures, clarifications and comments
added, typos corrected and one reference adde
A Pair of Dopamine Neurons Target the D1-Like Dopamine Receptor DopR in the Central Complex to Promote Ethanol-Stimulated Locomotion in Drosophila
Dopamine is a mediator of the stimulant properties of drugs of abuse, including ethanol, in mammals and in the fruit fly Drosophila. The neural substrates for the stimulant actions of ethanol in flies are not known. We show that a subset of dopamine neurons and their targets, through the action of the D1-like dopamine receptor DopR, promote locomotor activation in response to acute ethanol exposure. A bilateral pair of dopaminergic neurons in the fly brain mediates the enhanced locomotor activity induced by ethanol exposure, and promotes locomotion when directly activated. These neurons project to the central complex ellipsoid body, a structure implicated in regulating motor behaviors. Ellipsoid body neurons are required for ethanol-induced locomotor activity and they express DopR. Elimination of DopR blunts the locomotor activating effects of ethanol, and this behavior can be restored by selective expression of DopR in the ellipsoid body. These data tie the activity of defined dopamine neurons to D1-like DopR-expressing neurons to form a neural circuit that governs acute responding to ethanol
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