7,525 research outputs found
Structure of Lanczos-Lovelock Lagrangians in Critical Dimensions
The Lanczos-Lovelock models of gravity constitute the most general theories
of gravity in D dimensions which satisfy (a) the principle of of equivalence,
(b) the principle of general co-variance, and (c) have field equations
involving derivatives of the metric tensor only up to second order. The mth
order Lanczos-Lovelock Lagrangian is a polynomial of degree m in the curvature
tensor. The field equations resulting from it become trivial in the critical
dimension and the action itself can be written as the integral of an
exterior derivative of an expression involving the vierbeins, in the
differential form language. While these results are well known, there is some
controversy in the literature as to whether the Lanczos-Lovelock Lagrangian
itself can be expressed as a total divergence of quantities built only from the
metric and its derivatives (without using the vierbeins) in . We settle
this issue by showing that this is indeed possible and provide an algorithm for
its construction. In particular, we demonstrate that, in two dimensions, for a doublet of functions which
depends only on the metric and its first derivatives. We explicitly construct
families of such R^j -s in two dimensions. We also address related questions
regarding the Gauss-Bonnet Lagrangian in . Finally, we demonstrate the
relation between the Chern-Simons form and the mth order Lanczos-Lovelock
Lagrangian.Comment: 15 pages, no figure
Self-phoretic oscillatory motion in a harmonic trap
We consider the motion of a harmonically trapped overdamped particle, which
is submitted to a self-phoretic force, that is proportional to the gradient of
a diffusive field for which the particle itself is the source. In agreement
with existing results for free particles or particles in a bounded domain, we
find that the system exhibits a transition between an immobile phase, where the
particle stays at the center of the trap, and an oscillatory state. We perform
an exact analysis giving access to the bifurcation threshold, as well as the
frequency of oscillations and their amplitude near the threshold. Our analysis
also characterizes the shape of two-dimensional oscillations, that take place
along a circle or a straight line. Our results are confirmed by numerical
simulations.Comment: 10 pages 8 figure
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Observation of two-wave structure in strongly nonlinear dissipative granular chains
In a strongly nonlinear viscous granular chain under conditions of loading
that exclude stationary waves (e.g., impact by a single grain) we observe a
pulse that consists of two interconnected but distinct parts. One is a leading
narrow "primary pulse" with properties similar to a solitary wave in a "sonic
vacuum." It arises from strong nonlinearity and discreteness in the absence of
dissipation, but now decays due to viscosity. The other is a broad, much more
persistent shock-like "secondary pulse" trailing the primary pulse and caused
by viscous dissipation. The medium behind the primary pulse is transformed from
a "sonic vacuum" to a medium with finite sound speed. When the rapidly decaying
primary pulse dies, the secondary pulse continues to propagate in the "sonic
vacuum," with an oscillatory front if the viscosity is relatively small, until
its eventual (but very slow) disintegration. Beyond a critical viscosity there
is no separation of the two pulses, and the dissipation and nonlinearity
dominate the shock-like attenuating pulse which now exhibits a nonoscillatory
front
The Domination Number of Grids
In this paper, we conclude the calculation of the domination number of all
grid graphs. Indeed, we prove Chang's conjecture saying that for
every , .Comment: 12 pages, 4 figure
Spacetime Symmetries and Kepler's Third Law
The curved spacetime geometry of a system of two point masses moving on a
circular orbit has a helical symmetry. We show how Kepler's third law for
circular motion, and its generalization in post-Newtonian theory, can be
recovered from a simple, covariant condition on the norm of the associated
helical Killing vector field. This unusual derivation can be used to illustrate
some concepts of prime importance in a general relativity course, including
those of Killing field, covariance, coordinate dependence, and gravitational
redshift.Comment: 11 pages, 3 figures; minor changes and text improvements; matches
version to appear in Class. Quant. Gra
ResÃduo desidratado de vitivinÃcolas do Vale do São Francisco associado a diferentes fontes energéticas na alimentação de ovinos: desempenho animal.
Objetivou-se com a realização deste trabalho avaliar o ganho de peso e conversão alimentar em ovinos confinados recebendo dietas contendo resÃduo de vitivinÃcolas associado a diferentes fontes energéticas. Foram utilizados 18 ovinos sem padrão racial definido, não castrados, com peso médio de 23 kg e oito meses de idade. O perÃodo experimental constou de 63 dias, sendo as dietas compostas de 50% de resÃduo de vitivinÃcolas e 50% de concentrados energéticos: grão de milho moÃdo (Zea mays), raspa de mandioca (Manihot esculenta) enriquecida com 1,8% de uréia e farelo de palma forrageira (Opuntia ficus) enriquecido com 1,1% de uréia. Para determinação do ganho de peso os animais foram pesados no inÃcio do experimento e a cada sete dias e, para coversão alimentar foi feita a relação entre o consumo de matéria seca e ganho de peso total num perÃodo de 63 dias. Os ganhos de peso médio diários foram de 117, 71 e 132g; a conversão alimentar 9,50, 13,28 e 11,30, respectivamente para as combinações resÃduo e grão de milho moÃdo, raspa de mandioca e farelo de palma. As médias diárias de ganho de peso vivo obtido pelos ovinos ao longo do perÃodo de engorda revelaram um bom potencial forrageiro do resÃduo de vitivinÃcolas combinado as diferentes fontes energéticas
Complex microwave conductivity of Na-DNA powders
We report the complex microwave conductivity, , of
Na-DNA powders, which was measured from 80 K to 300 K by using a microwave
cavity perturbation technique. We found that the magnitude of near
room temperature was much larger than the contribution of the surrounding water
molecules, and that the decrease of with decreasing temperature was
sufficiently stronger than that of the conduction of counterions. These results
clearly suggest that the electrical conduction of Na-DNA is intrinsically
semiconductive.Comment: 16 pages, 7 figure
Anomalous magnetic moment in parity-conserving QED3
In this article we derive the anomalous magnetic moment of fermions in
(2+1)-dimensional parity-conserving QED3, in the presence of an externally
applied constant magnetic field. We use a spectral representation of the photon
propagator to avoid infrared divergences. We also discuss the scaling with the
magnetic field intensity in the case of strong external fields, where there is
dynamical mass generation for fermions induced by the magnetic field itself
(magnetic catalysis). The results of this paper may be of relevance to the
physics of high-temperature superconductors.Comment: 27 pages LATEX, three eps figures incorporate
Lectures on the functional renormalization group method
These introductory notes are about functional renormalization group equations
and some of their applications. It is emphasised that the applicability of this
method extends well beyond critical systems, it actually provides us a general
purpose algorithm to solve strongly coupled quantum field theories. The
renormalization group equation of F. Wegner and A. Houghton is shown to resum
the loop-expansion. Another version, due to J. Polchinski, is obtained by the
method of collective coordinates and can be used for the resummation of the
perturbation series. The genuinely non-perturbative evolution equation is
obtained in a manner reminiscent of the Schwinger-Dyson equations. Two variants
of this scheme are presented where the scale which determines the order of the
successive elimination of the modes is extracted from external and internal
spaces. The renormalization of composite operators is discussed briefly as an
alternative way to arrive at the renormalization group equation. The scaling
laws and fixed points are considered from local and global points of view.
Instability induced renormalization and new scaling laws are shown to occur in
the symmetry broken phase of the scalar theory. The flattening of the effective
potential of a compact variable is demonstrated in case of the sine-Gordon
model. Finally, a manifestly gauge invariant evolution equation is given for
QED.Comment: 47 pages, 11 figures, final versio
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