5,856 research outputs found
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
The spinfoam approach to quantum gravity rests on a "quantization" of BF
theory using 2-complexes and group representations. We explain why, in
dimension three and higher, this "spinfoam quantization" must be amended to be
made consistent with the gauge symmetries of discrete BF theory. We discuss a
suitable generalization, called "cellular quantization", which (1) is finite,
(2) produces a topological invariant, (3) matches with the properties of the
continuum BF theory, (4) corresponds to its loop quantization. These results
significantly clarify the foundations - and limitations - of the spinfoam
formalism, and open the path to understanding, in a discrete setting, the
symmetry-breaking which reduces BF theory to gravity.Comment: 6 page
Algorithmic correspondence and completeness in modal logic. I. The core algorithm SQEMA
Modal formulae express monadic second-order properties on Kripke frames, but
in many important cases these have first-order equivalents. Computing such
equivalents is important for both logical and computational reasons. On the
other hand, canonicity of modal formulae is important, too, because it implies
frame-completeness of logics axiomatized with canonical formulae.
Computing a first-order equivalent of a modal formula amounts to elimination
of second-order quantifiers. Two algorithms have been developed for
second-order quantifier elimination: SCAN, based on constraint resolution, and
DLS, based on a logical equivalence established by Ackermann.
In this paper we introduce a new algorithm, SQEMA, for computing first-order
equivalents (using a modal version of Ackermann's lemma) and, moreover, for
proving canonicity of modal formulae. Unlike SCAN and DLS, it works directly on
modal formulae, thus avoiding Skolemization and the subsequent problem of
unskolemization. We present the core algorithm and illustrate it with some
examples. We then prove its correctness and the canonicity of all formulae on
which the algorithm succeeds. We show that it succeeds not only on all
Sahlqvist formulae, but also on the larger class of inductive formulae,
introduced in our earlier papers. Thus, we develop a purely algorithmic
approach to proving canonical completeness in modal logic and, in particular,
establish one of the most general completeness results in modal logic so far.Comment: 26 pages, no figures, to appear in the Logical Methods in Computer
Scienc
Disordered Bose Einstein Condensates with Interaction in One Dimension
We study the effects of random scatterers on the ground state of the
one-dimensional Lieb-Liniger model of interacting bosons on the unit interval
in the Gross-Pitaevskii regime. We prove that Bose Einstein condensation
survives even a strong random potential with a high density of scatterers. The
character of the wave function of the condensate, however, depends in an
essential way on the interplay between randomness and the strength of the
two-body interaction. For low density of scatterers or strong interactions the
wave function extends over the whole interval. High density of scatterers and
weak interaction, on the other hand, leads to localization of the wave function
in a fragmented subset of the interval
Evaporation limited loading of an atom trap
Recently, we have experimentally demonstrated a continuous loading mechanism
for an optical dipole trap from a guided atomic beam [1]. The observed
evolution of the number of atoms and temperature in the trap are consequences
of the unusual trap geometry. In the present paper, we develop a model based on
a set of rate equations to describe the loading dynamics of such a mechanism.
We consider the collision statistics in the non-uniform trap potential that
leads to twodimensional evaporation. The comparison between the resulting
computations and experimental data allows to identify the dominant loss process
and suggests ways to enhance the achievable steady-state atom number.
Concerning subsequent evaporative cooling, we find that the possibility of
controlling axial and radial confinement independently allows faster
evaporation ramps compared to single beam optical dipole traps.Comment: 10 pages, 7 figure
ArgR is an essential local transcriptional regulator of the arcABC-operon in Streptococcus suis and crucial for biological fitness in acidic environment
Streptococcus suis is one of the most important pathogens in pigs and can also cause severe infections in humans. Despite its clinical relevance very little is known about the factors contributing to its virulence. Recently, we identified a new putative virulence factor in Streptococcus suis, the arginine deiminase system (ADS), an arginine catabolic enzyme system encoded by the arcABC-operon, which enables Streptococcus suis to survive in acidic environment. In this study, we focused on ArgR, an ADS associated regulator belonging to the ArgR/AhrC arginine repressor family. Using an argR knock-out strain we could show that ArgR is essential for arcABC-operon expression and necessary for the biological fitness of Streptococcus suis. By cDNA expression microarray analyses and quantitative real time RT-PCR we found that the arcABC-operon is the only gene cluster regulated by ArgR, which is in contrast to many other bacteria. Reporter gene analysis with gfp under the control of the arcABC promoter demonstrated that ArgR is able to activate the arcABC promoter. Electrophoretic mobility shift assays with fragments of the arcABC promoter and recombinant ArgR, and chromatin immunoprecipitation with antibodies directed against ArgR revealed that ArgR interacts with the arcABC promoter in vitro and in vivo by binding to a region from -147 to 72 bp upstream of the transcriptional start point. Overall our results show that in Streptococcus suis ArgR is an essential, system specific transcriptional regulator of the ADS directly interacting with the arcABC promoter in vivo
Bubble divergences from cellular cohomology
We consider a class of lattice topological field theories, among which are
the weak-coupling limit of 2d Yang-Mills theory, the Ponzano-Regge model of 3d
quantum gravity and discrete BF theory, whose dynamical variables are flat
discrete connections with compact structure group on a cell 2-complex. In these
models, it is known that the path integral measure is ill-defined in general,
because of a phenomenon called `bubble divergences'. A common expectation is
that the degree of these divergences is given by the number of `bubbles' of the
2-complex. In this note, we show that this expectation, although not realistic
in general, is met in some special cases: when the 2-complex is simply
connected, or when the structure group is Abelian -- in both cases, the
divergence degree is given by the second Betti number of the 2-complex.Comment: 5 page
Ballistic Electron Quantum Transport in Presence of a Disordered Background
Effect of a complicated many-body environment is analyzed on the electron
random scattering by a 2D mesoscopic open ballistic structure. A new mechanism
of decoherence is proposed. The temperature of the environment is supposed to
be zero whereas the energy of the incoming particle can be close to or
somewhat above the Fermi surface in the environment. The single-particle
doorway resonance states excited in the structure via external channels are
damped not only because of escape through such channels but also due to the
ulterior population of the long-lived environmental states. Transmission of an
electron with a given incoming through the structure turns out to be
an incoherent sum of the flow formed by the interfering damped doorway
resonances and the retarded flow of the particles re-emitted into the structure
by the environment. Though the number of the particles is conserved in each
individual event of transmission, there exists a probability that some part of
the electron's energy can be absorbed due to environmental many-body effects.
In such a case the electron can disappear from the resonance energy interval
and elude observation at the fixed transmission energy thus resulting
in seeming loss of particles, violation of the time reversal symmetry and, as a
consequence, suppression of the weak localization. The both decoherence and
absorption phenomena are treated within the framework of a unit microscopic
model based on the general theory of the resonance scattering. All the effects
discussed are controlled by the only parameter: the spreading width of the
doorway resonances, that uniquely determines the decoherence rateComment: 7 pages, 1 figure. The published version. A figure has been added;
the list of references has been improved. Some explanatory remarks have been
include
Bubble divergences: sorting out topology from cell structure
We conclude our analysis of bubble divergences in the flat spinfoam model. In
[arXiv:1008.1476] we showed that the divergence degree of an arbitrary
two-complex Gamma can be evaluated exactly by means of twisted cohomology.
Here, we specialize this result to the case where Gamma is the two-skeleton of
the cell decomposition of a pseudomanifold, and sharpen it with a careful
analysis of the cellular and topological structures involved. Moreover, we
explain in detail how this approach reproduces all the previous powercounting
results for the Boulatov-Ooguri (colored) tensor models, and sheds light on
algebraic-topological aspects of Gurau's 1/N expansion.Comment: 19 page
Power sensitivity and algebraic technique for evaluation of penetration level of photovoltaic on DC link of VSC HVDC transmission
Abstract: With the increased advent of VSC HVDC with long DC transmission link in power systems, situations have arisen and will be even more frequent in the future, where several distributed generation will be connected on the DC-link for more power transfer capability. As penetration level increases, there is need to predict the limit before violation of voltage and power instability on the DC transmission link and ensure that it does not interfere with the main VSC HVDC system control. In this paper, power sensitivity and algebraic technique is proposed to predict the maximum DG penetration that can be accepted at a particular location on the dc link of VSC HVDC transmission system before violation of voltage and power stabilit
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