2,097 research outputs found
Noether's Symmetry Theorem for Variational and Optimal Control Problems with Time Delay
We extend the DuBois-Reymond necessary optimality condition and Noether's
symmetry theorem to the time delay variational setting. Both Lagrangian and
Hamiltonian versions of Noether's theorem are proved, covering problems of the
calculus of variations and optimal control with delays.Comment: This is a preprint of a paper whose final and definite form will
appear in the international journal Numerical Algebra, Control and
Optimization (NACO). Paper accepted for publication 15-March-201
Mathematical retroreflectors
Retroreflectors are optical devices that reverse the direction of incident
beams of light. Here we present a collection of billiard type retroreflectors
consisting of four objects; three of them are asymptotically perfect
retroreflectors, and the fourth one is a retroreflector which is very close to
perfect. Three objects of the collection have recently been discovered and
published or submitted for publication. The fourth object - notched angle - is
a new one; a proof of its retroreflectivity is given.Comment: 32 pages, 19 figure
On the two-dimensional rotational body of maximal Newtonian resistance
We investigate, by means of computer simulations, shapes of nonconvex bodies
that maximize resistance to their motion through a rarefied medium, considering
that bodies are moving forward and at the same time slowly rotating. A
two-dimensional geometric shape that confers to the body a resistance very
close to the theoretical supremum value is obtained, improving previous
results.Comment: This is a preprint version of the paper published in J. Math. Sci.
(N. Y.), Vol. 161, no. 6, 2009, 811--819. DOI:10.1007/s10958-009-9602-
Molecular alterations of KIT and PDGFRA in GISTs: evaluation of a Portuguese series
To assess KIT and PDGFRA mutations frequencies in a Portuguese series of gastrointestinal stromal tumours (GISTs)
The Possibility of Thermal Instability in Early-Type Stars Due to Alfven Waves
It was shown by dos Santos et al. the importance of Alfv\'en waves to explain
the winds of Wolf-Rayet stars. We investigate here the possible importance of
Alfv\'en waves in the creation of inhomogeneities in the winds of early-type
stars. The observed infrared emission (at the base of the wind) of early-type
stars is often larger than expected. The clumping explains this characteristic
in the wind, increasing the mean density and hence the emission measure, making
possible to understand the observed infrared, as well as the observed
enhancement in the blue wing of the line. In this study, we
investigate the formation of these clumps a via thermal instability. The
heat-loss function used, , includes physical processes such as:
emission of (continuous and line) recombination radiation; resonance line
emission excited by electron collisions; thermal bremsstrahlung; Compton
heating and cooling; and damping of Alfv\'en waves. As a result of this
heat-loss function we show the existence of two stable equilibrium regions. The
stable equilibrium region at high temperature is the diffuse medium and at low
temperature the clumps. Using this reasonable heat-loss function, we show that
the two stable equilibrium regions can coexist over a narrow range of pressures
describing the diffuse medium and the clumps.Comment: 21 pages (psfig.sty), 5 figures (included), ApJ accepted. Also
available at http://www.iagusp.usp.br/preprints/preprint.htm
Modeling the high-energy emission in GRB 110721A and implications on the early multiwavelength and polarimetric observations
GRB 110721A was detected by the Gamma-ray Burst Monitor and the Large Area
Telescope (LAT) onboard the Fermi satellite and the Gamma-ray Burst Polarimeter
onboard the IKAROS solar mission. Previous analysis done of this burst showed:
i) a linear polarization signal with position angle stable () and high degree of , ii) an extreme peak
energy of a record-breaking at 152 MeV, and iii) a subdominant prompt
thermal component observed right after the onset of this burst. In this paper,
the LAT data around the reported position of GRB 110721A are analysed with the
most recent software and then, the LAT light curve above 100 MeV was obtained.
The LAT light curve is modelled in terms of adiabatic early-afterglow external
shocks when the outflow propagates into a stellar wind. Additionally, we
discuss the possible origins and also study the implications of the
early-afterglow external shocks on the extreme peak energy observed at 152
MeV, the polarization observations and the subdominant prompt thermal
component.Comment: 9 pages and one figure. Accepted for publication in Ap
Active Galactic Nuclei under the scrutiny of CTA
Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer
excellent conditions for efficient particle acceleration in internal and
external shocks, turbulence, and magnetic reconnection events. The jets as well
as particle accelerating regions close to the supermassive black holes
(hereafter SMBH) at the intersection of plasma inflows and outflows, can
produce readily detectable very high energy gamma-ray emission. As of now, more
than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the
present ground-based gamma-ray telescopes, which represents more than one third
of the cosmic sources detected so far in the VHE gamma-ray regime. The future
Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the
VHE range by about one order of magnitude, shedding new light on AGN population
studies, and AGN classification and unification schemes. CTA will be a unique
tool to scrutinize the extreme high-energy tail of accelerated particles in
SMBH environments, to revisit the central engines and their associated
relativistic jets, and to study the particle acceleration and emission
mechanisms, particularly exploring the missing link between accretion physics,
SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an
extremely rewarding observing program which will inform us about the inner
workings and evolution of AGN. Furthermore these AGN are bright beacons of
gamma-rays which will allow us to constrain the extragalactic infrared and
optical backgrounds as well as the intergalactic magnetic field, and will
enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure
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