1,101 research outputs found
Distributed Systems of Intersecting Branes at Arbitrary Angles
A `reduced' action formulation for a general class of the supergravity
solutions, corresponding to the `marginally' bound `distributed' systems of
various types of branes at arbitrary angles, is developed. It turns out that
all the information regarding the classical features of such solutions is
encoded in a first order Lagrangian (the `reduced' Lagrangian) corresponding to
the desired geometry of branes. The marginal solution for a system of such
distributions (for various distribution functions) span an dimensional
submanifold of the fields' configuration (target) space, parametrised by a set
of independent harmonic functions on the transverse space. This
submanifold, which we call it as the `-surface', is a null surface with
respect to a metric on the configuration space, which is defined by the reduced
Lagrangian. The equations of motion then transform to a set of equations
describing the embedding of a null geodesic surface in this space, which is
identified as the -surface. Using these facts, we present a very simple
derivation of the conventional orthogonal solutions together with their
intersection rules. Then a new solution for a (distributed) pair of -branes
at SU(2) angles in dimensions is derived.Comment: Latex file, 58 pages, no figures, 5 tables, This revision contains
some minor changes of the original version including those of the title,
abstract and referrences. Some comments are adde
Public behaviour in response to the Covid-19 pandemic: Understanding the role of group processes
Background
In the absence of a vaccine, behaviour by the public is key to the response to the Covid-19 pandemic. Yet, as with other types of crises and emergencies, there have been doubts about the extent to which the public are able to engage effectively with the required behaviour. These doubts are based on outdated models of group psychology.
Aims and argument
We analyse the role of group processes in the Covid-19 pandemic in three domains: recognition of threat; adherence by the public to the required public health behaviours (and the factors that increase such adherence); and actions of the many community mutual aid groups that arose during lockdown. In each case, we draw upon the accumulated research on behaviour in emergencies and disasters as well as the latest findings in relation to the Covid-19 pandemic to show that explanations in terms of social identity processes make better sense of the patterns of evidence than alternative explanations.
Conclusion
If behaviour in the pandemic is a function of mutable group processes rather than fixed tendencies, then behavioural change is possible. There was evidence of significant change in behaviour from the public, particularly in the early days of the pandemic. Understanding the role of group processes means we can help design more effective interventions to support collective resilience in the public in the face of the pandemic and other threats. We draw out from the evidence a set of recommendations on facilitating the public response to Covid-19 by harnessing group processes
Hamilton's equations for a fluid membrane: axial symmetry
Consider a homogenous fluid membrane, or vesicle, described by the
Helfrich-Canham energy, quadratic in the mean curvature. When the membrane is
axially symmetric, this energy can be viewed as an `action' describing the
motion of a particle; the contours of equilibrium geometries are identified
with particle trajectories. A novel Hamiltonian formulation of the problem is
presented which exhibits the following two features: {\it (i)} the second
derivatives appearing in the action through the mean curvature are accommodated
in a natural phase space; {\it (ii)} the intrinsic freedom associated with the
choice of evolution parameter along the contour is preserved. As a result, the
phase space involves momenta conjugate not only to the particle position but
also to its velocity, and there are constraints on the phase space variables.
This formulation provides the groundwork for a field theoretical generalization
to arbitrary configurations, with the particle replaced by a loop in space.Comment: 11 page
An augmented lagrangian method for sparse SAR imaging
In this paper, we present a solution to the constrained l1-norm minimization problem for sparse SAR imaging. The technique we present relies on recent advances in the solution of optimization problems, based on Augmented Lagrangian Methods (ALMs), namely the Alternating Direction Method of Multipliers. Here, we present an application of C-SALSA (an ALM for constrained optimization problems) to SAR imaging, and introduce a new weighting scheme to improve the sparsity of the reconstructions. We then compare the performances of several techniques to understand the effectiveness of ALMs in the context of SAR imaging
Axially symmetric membranes with polar tethers
Axially symmetric equilibrium configurations of the conformally invariant
Willmore energy are shown to satisfy an equation that is two orders lower in
derivatives of the embedding functions than the equilibrium shape equation, not
one as would be expected on the basis of axial symmetry. Modulo a translation
along the axis, this equation involves a single free parameter c.If c\ne 0, a
geometry with spherical topology will possess curvature singularities at its
poles. The physical origin of the singularity is identified by examining the
Noether charge associated with the translational invariance of the energy; it
is consistent with an external axial force acting at the poles. A one-parameter
family of exact solutions displaying a discocyte to stomatocyte transition is
described.Comment: 13 pages, extended and revised version of Non-local sine-Gordon
equation for the shape of axi-symmetric membrane
The Jang equation, apparent horizons, and the Penrose inequality
The Jang equation in the spherically symmetric case reduces to a first order
equation. This permits an easy analysis of the role apparent horizons play in
the (non)existence of solutions. We demonstrate that the proposed derivation of
the Penrose inequality based on the Jang equation cannot work in the
spherically symmetric case. Thus it is fruitless to apply this method, as it
stands, to the general case. We show also that those analytic criteria for the
formation of horizons that are based on the use of the Jang equation are of
limited validity for the proof of the trapped surface conjecture.Comment: minor misprints correcte
Geometry of Deformations of Relativistic Membranes
A kinematical description of infinitesimal deformations of the worldsheet
spanned in spacetime by a relativistic membrane is presented. This provides a
framework for obtaining both the classical equations of motion and the
equations describing infinitesimal deformations about solutions of these
equations when the action describing the dynamics of this membrane is
constructed using {\it any} local geometrical worldsheet scalars. As examples,
we consider a Nambu membrane, and an action quadratic in the extrinsic
curvature of the worldsheet.Comment: 20 pages, Plain Tex, sign errors corrected, many new references
added. To appear in Physical Review
Modelovanje pražnjenja creva evropskog brancina dicentrarachus labrax l.
Digestion physiology of an animal is important since it is essential source of information for aquaculture and also it helps to outline the rule of such animal in the aquatic food web. Especially it is vital for the management issues of the living resources concerning multispecies VPA. Bearing in mind, sea bass is a one of the most important sea fish having high salinity and temperature tolerance and commercial value not only in Turkey, but in the European countries as well. It has growing culturing potential in Turkey and Europe, but not much detailed studies concerned feeding and digestion physiology have been performed yet.
In this study, digestion physiology; gastric emptying in Dicentrarchus labrax force fed on artificial (formulated) food will be investigated. Factors affecting gastric emptying rate (GER) and time (GET) in sea bass will be studied. An attempt will also be made to model GER and GET in sea bass. Food consumption and feeding periodicity and return of appetite of sea bass will also be worked out
The String Deviation Equation
The relative motion of many particles can be described by the geodesic
deviation equation. This can be derived from the second covariant variation of
the point particle's action. It is shown that the second covariant variation of
the string action leads to a string deviation equation.Comment: 18 pages, some small changes, no tables or diagrams, LaTex2
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