15,761 research outputs found
Force indeterminacy in the jammed state of hard disks
Granular packings of hard discs are investigated by means of contact dynamics
which is an appropriate technique to explore the allowed force-realizations in
the space of contact forces. Configurations are generated for given values of
the friction coefficient, and then an ensemble of equilibrium forces is found
for fixed contacts. We study the force fluctuations within this ensemble. In
the limit of zero friction the fluctuations vanish in accordance with the
isostaticity of the packing. The magnitude of the fluctuations has a
non-monotonous friction dependence. The increase for small friction can be
attributed to the opening of the angle of the Coulomb cone, while the decrease
as friction increases is due to the reduction of connectivity of the
contact-network, leading to local, independent clusters of indeterminacy. We
discuss the relevance of indeterminacy to packings of deformable particles and
to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde
Axisymmetric Stationary Solutions as Harmonic Maps
We present a method for generating exact solutions of Einstein equations in
vacuum using harmonic maps, when the spacetime possesses two commutating
Killing vectors. This method consists in writing the axisymmetric stationry
Einstein equations in vacuum as a harmonic map which belongs to the group
SL(2,R), and decomposing it in its harmonic "submaps". This method provides a
natural classification of the solutions in classes (Weil's class, Lewis' class
etc).Comment: 17 TeX pages, one table,( CINVESTAV- preprint 12/93
Report of the panel on lithospheric structure and evolution, section 3
The panel concluded that NASA can contribute to developing a refined understanding of the compositional, structural, and thermal differences between continental and oceanic lithosphere through a vigorous program in solid Earth science with the following objectives: determine the most fundamental geophysical property of the planet; determine the global gravity field to an accuracy of a few milliGals at wavelengths of 100 km or less; determine the global lithospheric magnetic field to a few nanoTeslas at a wavelength of 100 km; determine how the lithosphere has evolved to its present state via acquiring geologic remote sensing data over all the continents
An Exact Solution for Static Scalar Fields Coupled to Gravity in -Dimensions
We obtain an exact solution for the Einstein's equations with cosmological
constant coupled to a scalar, static particle in static, "spherically"
symmetric background in 2+1 dimensions.Comment: 9 pages. Replaced by a revised versio
Antiretroviral therapy and HIV-associated cancers: Antiangiogenic effect of efavirenz on chick chorioallantoic membrane
Purpose: To investigate the anti-angiogenic effect of efavirenz in chick chorioallantoic membrane (CAM).Methods: Fertile eggs of the domestic fowl (Gallus gallus, variant Domesticus) were used as the in vivo vascular test environment. Experimental groups were VEGF, thalidomide (negative control), efavirenz (test drug), control 1 (treated with diluent) and control 2 (no treatment). CAM photographs were taken using a microscope camera. CAM blood vessels were scored as either âpresentâ or âabsentâ.Results: There was association between treatment drugs and growth of CAM blood vessels (p < 0.05) as follows: control 1 â 100 %, control 2 â 67 %, VEGF - 83 %, thalidomide - 17 %, and efavirenz - 0 % (n = 30). Although efavirenz had absolute anti-angiogenicity and presented as a potential anti-cancer drug, there was no significant difference between the anti-angiogenicity of efavirenz (100 %) and thalidomide (83 %) (F = 1.000, p = 0.341).Conclusion: Being a first-line drug in both HAART and combination treatment of HIV-1, efavirenz may be responsible for spontaneous resolution of secondary cancers in HIV-infected patients on HAART regimen.Keywords: Efavirenz, HIV, Cancers, Angiogenesis, Chick chorioallantoic membran
The full integration of black hole solutions to symmetric supergravity theories
We prove that all stationary and spherical symmetric black hole solutions to
theories with symmetric target spaces are integrable and we provide an explicit
integration method. This exact integration is based on the description of black
hole solutions as geodesic curves on the moduli space of the theory when
reduced over the time-like direction. These geodesic equations of motion can be
rewritten as a specific Lax pair equation for which mathematicians have
provided the integration algorithms when the initial conditions are described
by a diagonalizable Lax matrix. On the other hand, solutions described by
nilpotent Lax matrices, which originate from extremal regular (small) D = 4
black holes can be obtained as suitable limits of solutions obtained in the
diagonalizable case, as we show on the generating geodesic (i.e. most general
geodesic modulo global symmetries of the D = 3 model) corresponding to regular
(and small) D = 4 black holes. As a byproduct of our analysis we give the
explicit form of the Wick rotation connecting the orbits of BPS and non-BPS
solutions in maximally supersymmetric supergravity and its STU truncation.Comment: 27 pages, typos corrected, references added, 1 figure added,
Discussion on black holes and the generating geodesic significantly extended.
Statement about the relation between the D=3 geodesics from BPS and non-BPS
extreme black holes made explicit by defining the Wick rotation mapping the
corresponding orbit
Spherically symmetric Yang-Mills solutions in a (4+n)- dimensional space-time
We consider the Einstein-Yang-Mills Lagrangian in a (4+n)-dimensional
space-time. Assuming the matter and metric fields to be independent of the n
extra coordinates, a spherical symmetric Ansatz for the fields leads to a set
of coupled ordinary differential equations. We find that for n > 1 only
solutions with either one non-zero Higgs field or with all Higgs fields
constant exist. We construct the analytic solutions which fulfill this
conditions for arbitrary n, namely the Einstein-Maxwell-dilaton solutions. We
also present generic solutions of the effective 4-dimensional
Einstein-Yang-Mills-Higgs-dilaton model, which possesses n Higgs triplets
coupled in a specific way to n independent dilaton fields. These solutions are
the abelian Einstein-Maxwell- dilaton solutions and analytic non-abelian
solutions, which have diverging Higgs fields. In addition, we construct
numerically asymptotically flat and finite energy solutions for n=2.Comment: 15 Latex pages, 4 eps figures; v2: discussion of results revisite
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