7,226 research outputs found
Modal Logics with Hard Diamond-free Fragments
We investigate the complexity of modal satisfiability for certain
combinations of modal logics. In particular we examine four examples of
multimodal logics with dependencies and demonstrate that even if we restrict
our inputs to diamond-free formulas (in negation normal form), these logics
still have a high complexity. This result illustrates that having D as one or
more of the combined logics, as well as the interdependencies among logics can
be important sources of complexity even in the absence of diamonds and even
when at the same time in our formulas we allow only one propositional variable.
We then further investigate and characterize the complexity of the
diamond-free, 1-variable fragments of multimodal logics in a general setting.Comment: New version: improvements and corrections according to reviewers'
comments. Accepted at LFCS 201
Hamiltonian Formulation of Open WZW Strings
Using a Hamiltonian approach, we construct the classical and quantum theory
of open WZW strings on a strip. (These are the strings which end on WZW
branes.) The development involves non-abelian generalized Dirichlet images in
an essential way. At the classical level, we find a new non-commutative
geometry in which the equal-time coordinate brackets are non-zero at the
world-sheet boundary, and the result is an intrinsically non-abelian effect
which vanishes in the abelian limit. Using the classical theory as a guide to
the quantum theory, we also find the operator algebra and the analogue of the
Knizhnik-Zamolodchikov equations for the the conformal field theory of open WZW
strings.Comment: 34 pages. Added an equation in Appendix C; some typos corrected.
Footnote b changed. Version to appear on IJMP
Memristive excitable cellular automata
The memristor is a device whose resistance changes depending on the polarity
and magnitude of a voltage applied to the device's terminals. We design a
minimalistic model of a regular network of memristors using
structurally-dynamic cellular automata. Each cell gets info about states of its
closest neighbours via incoming links. A link can be one 'conductive' or
'non-conductive' states. States of every link are updated depending on states
of cells the link connects. Every cell of a memristive automaton takes three
states: resting, excited (analog of positive polarity) and refractory (analog
of negative polarity). A cell updates its state depending on states of its
closest neighbours which are connected to the cell via 'conductive' links. We
study behaviour of memristive automata in response to point-wise and spatially
extended perturbations, structure of localised excitations coupled with
topological defects, interfacial mobile excitations and growth of information
pathways.Comment: Accepted to Int J Bifurcation and Chaos (2011
New Spin-Two Gauged Sigma Models and General Conformal Field Theory
Recently, we have studied the general Virasoro construction at one loop in
the background of the general non-linear sigma model. Here, we find the action
formulation of these new conformal field theories when the background sigma
model is itself conformal. In this case, the new conformal field theories are
described by a large class of new spin-two gauged sigma models. As examples of
the new actions, we discuss the spin-two gauged WZW actions, which describe the
conformal field theories of the generic affine-Virasoro construction, and the
spin-two gauged g/h coset constructions. We are able to identify the latter as
the actions of the local Lie h-invariant conformal field theories, a large
class of generically irrational conformal field theories with a local gauge
symmetry.Comment: LaTeX, 28 pages, references and clarifying remarks adde
The Ultraviolet Spectra of Active Galaxies With Double-Peaked Emission Lines
We present the results of UV spectroscopy of AGNs with double-peaked Balmer
emission lines. In 2/3 of the objects, the far-UV resonance lines are strong,
with single-peaked profiles resembling those of Seyfert galaxies. The Mg II
line is the only UV line with a double-peaked profile. In the remaining
objects, the far-UV resonance lines are relatively weak but still
single-peaked. The latter group also displays prominent UV absorption lines,
indicative of a low-ionization absorber. We interpret the difference in the
profiles of the emission lines as resulting from two different regions: a
dense, low-ionization accretion disk (the predominant source of the Balmer and
Mg II lines), and a lower density, higher-ionization wind (the predominant
source of the far-UV resonance lines). These results suggest a way of
connecting the double-peaked emitters with the greater AGN population: in
double-peaked emitters the accretion rate onto the black hole is low, making
the wind feeble and allowing the lines from the underlying disk to shine
through. This scenario also implies that in the majority of AGNs, the wind is
the source of the broad emission lines.Comment: To appear in "The Interplay among Black Holes, Stars and ISM in
Galactic Nuclei," IAU Coll. 222, eds. Storchi Bergmann, Ho, and Schmit
General Solution of the non-abelian Gauss law and non-abelian analogs of the Hodge decomposition
General solution of the non-abelian Gauss law in terms of covariant curls and
gradients is presented. Also two non-abelian analogs of the Hodge decomposition
in three dimensions are addressed. i) Decomposition of an isotriplet vector
field as sum of covariant curl and gradient with respect to an
arbitrary background Yang-Mills potential is obtained. ii) A decomposition of
the form which involves non-abelian
magnetic field of a new Yang-Mills potential C is also presented. These results
are relevant for duality transformation for non-abelian gauge fields.Comment: 6 pages, no figures, revte
Supergrassmannian and large N limit of quantum field theory with bosons and fermions
We study a large N_{c} limit of a two-dimensional Yang-Mills theory coupled
to bosons and fermions in the fundamental representation. Extending an approach
due to Rajeev we show that the limiting theory can be described as a classical
Hamiltonian system whose phase space is an infinite-dimensional
supergrassmannian. The linear approximation to the equations of motion and the
constraint yields the 't Hooft equations for the mesonic spectrum. Two other
approximation schemes to the exact equations are discussed.Comment: 24 pages, Latex; v.3 appendix added, typos corrected, to appear in
JM
Influence of detector motion in entanglement measurements with photons
We investigate how the polarization correlations of entangled photons
described by wave packets are modified when measured by moving detectors. For
this purpose, we analyze the Clauser-Horne-Shimony-Holt Bell inequality as a
function of the apparatus velocity. Our analysis is motivated by future
experiments with entangled photons designed to use satellites. This is a first
step towards the implementation of quantum information protocols in a global
scale
Jets in GRBs
In several GRBs afterglows, rapid temporal decay is observed which is
inconsistent with spherical (isotropic) blast-wave models. In particular, GRB
980519 had the most rapidly fading of the well-documented GRB afterglows, with
t^{-2.05\pm 0.04} in optical as well as in X-rays. We show that such temporal
decay is more consistent with the evolution of a jet after it slows down and
spreads laterally, for which t^{-p} decay is expected (where p is the index of
the electron energy distribution). Such a beaming model would relax the energy
requirements on some of the more extreme GRBs by a factor of several hundreds.
It is likely that a large fraction of the weak (or no) afterglow observations
are also due to the common occurrence of beaming in GRBs, and that their jets
have already transitioned to the spreading phase before the first afterglow
observations were made. With this interpretation, a universal value of p~2.5 is
consistent with all data.Comment: 4 page
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Recent pace of change in human impact on the world's ocean.
Humans interact with the oceans in diverse and profound ways. The scope, magnitude, footprint and ultimate cumulative impacts of human activities can threaten ocean ecosystems and have changed over time, resulting in new challenges and threats to marine ecosystems. A fundamental gap in understanding how humanity is affecting the oceans is our limited knowledge about the pace of change in cumulative impact on ocean ecosystems from expanding human activities - and the patterns, locations and drivers of most significant change. To help address this, we combined high resolution, annual data on the intensity of 14 human stressors and their impact on 21 marine ecosystems over 11 years (2003-2013) to assess pace of change in cumulative impacts on global oceans, where and how much that pace differs across the ocean, and which stressors and their impacts contribute most to those changes. We found that most of the ocean (59%) is experiencing significantly increasing cumulative impact, in particular due to climate change but also from fishing, land-based pollution and shipping. Nearly all countries saw increases in cumulative impacts in their coastal waters, as did all ecosystems, with coral reefs, seagrasses and mangroves at most risk. Mitigation of stressors most contributing to increases in overall cumulative impacts is urgently needed to sustain healthy oceans
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