1,284 research outputs found
On the reconstruction of planar lattice-convex sets from the covariogram
A finite subset of is said to be lattice-convex if is
the intersection of with a convex set. The covariogram of
is the function associating to each u \in
\integer^d the cardinality of . Daurat, G\'erard, and Nivat and
independently Gardner, Gronchi, and Zong raised the problem on the
reconstruction of lattice-convex sets from . We provide a partial
positive answer to this problem by showing that for and under mild extra
assumptions, determines up to translations and reflections. As a
complement to the theorem on reconstruction we also extend the known
counterexamples (i.e., planar lattice-convex sets which are not
reconstructible, up to translations and reflections) to an infinite family of
counterexamples.Comment: accepted in Discrete and Computational Geometr
Leading strategies in competitive on-line prediction
We start from a simple asymptotic result for the problem of on-line
regression with the quadratic loss function: the class of continuous
limited-memory prediction strategies admits a "leading prediction strategy",
which not only asymptotically performs at least as well as any continuous
limited-memory strategy but also satisfies the property that the excess loss of
any continuous limited-memory strategy is determined by how closely it imitates
the leading strategy. More specifically, for any class of prediction strategies
constituting a reproducing kernel Hilbert space we construct a leading
strategy, in the sense that the loss of any prediction strategy whose norm is
not too large is determined by how closely it imitates the leading strategy.
This result is extended to the loss functions given by Bregman divergences and
by strictly proper scoring rules.Comment: 20 pages; a conference version is to appear in the ALT'2006
proceeding
The Classical Exchange Algebra of AdS5 x S5 String Theory
The classical exchange algebra satisfied by the monodromy matrix of AdS5 x S5
string theory in the Green-Schwarz formulation is determined by using a
first-order Hamiltonian formulation and by adding to the Bena-Polchinski-Roiban
Lax connection terms proportional to constraints. This enables in particular to
show that the conserved charges of this theory are in involution. This result
is obtained for a general world-sheet metric. The same exchange algebra is
obtained within the pure spinor description of AdS5 x S5 string theory. These
results are compared to the one obtained by A. Mikhailov and S. Schaefer-Nameki
for the pure spinor formulation.Comment: 37 pages; v2: Result on Jacobi identity and Yang-Baxter equation
added in section 3.3; v3: References added; v4: Comparison with ref [12]
clarified in section 3.
Noncommutative Electromagnetism As A Large N Gauge Theory
We map noncommutative (NC) U(1) gauge theory on R^d_C X R^{2n}_{NC} to U(N ->
\infty) Yang-Mills theory on R^d_C, where R^d_C is a d-dimensional commutative
spacetime while R^{2n}_{NC} is a 2n-dimensional NC space. The resulting U(N)
Yang-Mills theory on R^d_C is equivalent to that obtained by the dimensional
reduction of (d+2n)-dimensional U(N) Yang-Mills theory onto R^d_C. We show that
the gauge-Higgs system (A_\mu,\Phi^a) in the U(N -> \infty) Yang-Mills theory
on R^d_C leads to an emergent geometry in the (d+2n)-dimensional spacetime
whose metric was determined by Ward a long time ago. In particular, the
10-dimensional gravity for d=4 and n=3 corresponds to the emergent geometry
arising from the 4-dimensional N=4 vector multiplet in the AdS/CFT duality. We
further elucidate the emergent gravity by showing that the gauge-Higgs system
(A_\mu,\Phi^a) in half-BPS configurations describes self-dual Einstein gravity.Comment: 25 pages; More clarifications, to appear in Eur. Phys. J.
A Field-theoretical Interpretation of the Holographic Renormalization Group
A quantum-field theoretical interpretation is given to the holographic RG
equation by relating it to a field-theoretical local RG equation which
determines how Weyl invariance is broken in a quantized field theory. Using
this approach we determine the relation between the holographic C theorem and
the C theorem in two-dimensional quantum field theory which relies on the
Zamolodchikov metric. Similarly we discuss how in four dimensions the
holographic C function is related to a conjectured field-theoretical C
function. The scheme dependence of the holographic RG due to the possible
presence of finite local counterterms is discussed in detail, as well as its
implications for the holographic C function. We also discuss issues special to
the situation when mass deformations are present. Furthermore we suggest that
the holographic RG equation may also be obtained from a bulk diffeomorphism
which reduces to a Weyl transformation on the boundary.Comment: 24 pages, LaTeX, no figures; references added, typos corrected,
paragraph added to section
Dilepton Spectra from Decays of Light Unflavored Mesons
The invariant mass spectrum of the and pairs
from decays of light unflavored mesons with masses below the -meson mass to final states containing along with a dilepton pair one
photon, one meson, and two mesons are calculated within the framework of the
effective meson theory. The results can be used for simulations of the dilepton
spectra in heavy-ion collisions and for experimental searches of dilepton meson
decays.Comment: 73 pages, 19 figures, 3 tables, REVTeX, new references adde
Assessment of health risks of policies
The assessment of health risks of policies is an inevitable, although challenging prerequisite for the inclusion of health considerations in political decision making. The aim of our project was to develop a so far missing methodological guide for the assessment of the complex impact structure of policies. The guide was developed in a consensual way based on experiences gathered during the assessment of specific national policies selected by the partners of an EU project. Methodological considerations were discussed and summarized in workshops and pilot tested on the EU Health Strategy for finalization. The combined tool, which includes a textual guidance and a checklist, follows the top-down approach, that is, it guides the analysis of causal chains from the policy through related health determinants and risk factors to health outcomes. The tool discusses the most important practical issues of assessment by impact level. It emphasises the transparent identification and prioritisation of factors, the consideration of the feasibility of exposure and outcome assessmentwith special focus on quantification. The developed guide provides useful methodological instructions for the comprehensive assessment of health risks of policies that can be effectively used in the health impact assessment of policy proposals.
Dilepton production in heavy ion collisions at intermediate energies
We present a unified description of the vector meson and dilepton production
in elementary and in heavy ion reactions. The production of vector mesons
() is described via the excitation of nuclear resonances ().
The theoretical framework is an extended vector meson dominance model (eVMD).
The treatment of the resonance decays with arbitrary spin is
covariant and kinematically complete. The eVMD includes thereby excited vector
meson states in the transition form factors. This ensures correct asymptotics
and provides a unified description of photonic and mesonic decays. The
resonance model is successfully applied to the production in
reactions. The same model is applied to the dilepton production in elementary
reactions (). Corresponding data are well reproduced. However, when
the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the
experimental dilepton spectra measured by the DLS Collaboration are
significantly underestimated at small invariant masses. As a possible solution
of this problem the destruction of quantum interference in a dense medium is
discussed. A decoherent emission through vector mesons decays enhances the
corresponding dilepton yield in heavy ion reactions. In the vicinity of the
-peak the reproduction of the data requires further a substantial
collisional broadening of the and in particular of the meson.Comment: 32 pages revtex, 19 figures, to appear in PR
Matter Outflows from AGN: A Unifying Model
We discuss a self-consistent unified model of the matter outflows from AGNs
based on a theoretical approach and involving data on AGN evolution and
structure. The model includes a unified geometry, two-phase gas dynamics,
radiation transfer, and absorption spectrum calculations in the UV and X-ray
bands. We briefly discuss several questions about the mass sources of the
flows, the covering factors, and the stability of the narrow absorption
details.Comment: 6 figures, accepted for publication in Astrophysics and Space Scienc
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha
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