947 research outputs found
An O(M(n) log n) algorithm for the Jacobi symbol
The best known algorithm to compute the Jacobi symbol of two n-bit integers
runs in time O(M(n) log n), using Sch\"onhage's fast continued fraction
algorithm combined with an identity due to Gauss. We give a different O(M(n)
log n) algorithm based on the binary recursive gcd algorithm of Stehl\'e and
Zimmermann. Our implementation - which to our knowledge is the first to run in
time O(M(n) log n) - is faster than GMP's quadratic implementation for inputs
larger than about 10000 decimal digits.Comment: Submitted to ANTS IX (Nancy, July 2010
Challenges for creating magnetic fields by cosmic defects
We analyse the possibility that topological defects can act as a source of
magnetic fields through the Harrison mechanism in the radiation era. We give a
detailed relativistic derivation of the Harrison mechanism at first order in
cosmological perturbations, and show that it is only efficient for temperatures
above T ~ 0.2 keV. Our main result is that the vector metric perturbations
generated by the defects cannot induce vorticity in the matter fluids at linear
order, thereby excluding the production of currents and magnetic fields. We
show that anisotropic stress in the matter fluids is required to source
vorticity and magnetic fields. Our analysis is relevant for any mechanism
whereby vorticity is meant to be transferred purely by gravitational
interactions, and thus would also apply to dark matter or neutrinos.Comment: 9 pages, 1 figure; minor corrections and additions; accepted for
publication in Physical Review
Ellipsoidal Universe Can Solve The CMB Quadrupole Problem
The recent three-year WMAP data have confirmed the anomaly concerning the low
quadrupole amplitude compared to the best-fit \Lambda CDM prediction. We show
that, allowing the large-scale spatial geometry of our universe to be
plane-symmetric with eccentricity at decoupling or order 10^{-2}, the
quadrupole amplitude can be drastically reduced without affecting higher
multipoles of the angular power spectrum of the temperature anisotropy.Comment: 4 pages, 2 figures, minor changes, reference added, to appear in
Phys. Rev. Let
Magnetic Helicity Generation from the Cosmic Axion Field
The coupling between a primordial magnetic field and the cosmic axion field
generates a helical component of the magnetic field around the time in which
the axion starts to oscillate. If the energy density of the seed magnetic field
is comparable to the energy density of the universe at that time, then the
resulting magnetic helicity is about |H_B| \simeq (10^{-20} G)^2 kpc and
remains constant after its generation. As a corollary, we find that the
standard properties of the oscillating axion remain unchanged even in the
presence of very strong magnetic fields.Comment: 6 pages, 2 figures. Accepted for publication in Phys. Rev. D. Minor
revisions and new references adde
Pulsar rotation measures and the magnetic structure of our Galaxy
We have obtained 63 rotation measures (RMs) from polarization observations of
southern pulsars, of which 54 are new measurements and 3 are varied from
previous values. The new pulsar RM data at high Galactic latitudes are mostly
consistent with the antisymmetric RM distribution found previously. For the
Galactic disc, evidence for a field reversal near the Perseus arm, and possibly
another beyond it, is presented. Inside the Solar Circle, in addition to the
two known field reversals in or near the Carina-Sagittartus arm and the
Crux-Scutum arm, a further reversal in the Norma arm is tentatively identified.
These reversals, together with the pitch angle derived from pulsar RM and
stellar polarization distributions, are consistent with bisymmetric spiral
(BSS) models for the large-scale magnetic field structure in the disc of our
Galaxy. However, discrimination between models is complicated by the presence
of smaller-scale irregularities in the magnetic field, as well as uncertainties
in the theoretical modelling.Comment: 10pages; 8 figures; Accepted by MNRA
Sharpenings of Li's criterion for the Riemann Hypothesis
Exact and asymptotic formulae are displayed for the coefficients
used in Li's criterion for the Riemann Hypothesis. For we obtain
that if (and only if) the Hypothesis is true,
(with and explicitly given, also for the case of more general zeta or
-functions); whereas in the opposite case, has a non-tempered
oscillatory form.Comment: 10 pages, Math. Phys. Anal. Geom (2006, at press). V2: minor text
corrections and updated reference
Nonequilibrium Electron Interactions in Metal Films
Ultrafast relaxation dynamics of an athermal electron distribution is
investigated in silver films using a femtosecond pump-probe technique with 18
fs pulses in off-resonant conditions. The results yield evidence for an
increase with time of the electron-gas energy loss rate to the lattice and of
the free electron damping during the early stages of the electron-gas
thermalization. These effects are attributed to transient alterations of the
electron average scattering processes due to the athermal nature of the
electron gas, in agreement with numerical simulations
CMB anisotropies due to cosmological magnetosonic waves
We study scalar mode perturbations (magnetosonic waves) induced by a helical
stochastic cosmological magnetic field and derive analytically the
corresponding cosmic microwave background (CMB) temperature and polarization
anisotropy angular power spectra. We show that the presence of a stochastic
magnetic field, or an homogeneous magnetic field, influences the acoustic
oscillation pattern of the CMB anisotropy power spectrum, effectively acting as
a reduction of the baryon fraction. We find that the scalar magnetic energy
density perturbation contribution to the CMB temperature anisotropy is small
compared to the contribution to the CMB -polarization anisotropy.Comment: 17 pages, references added, version accepted for publication in Phys.
Rev.
Optical Phonon Lasing in Semiconductor Double Quantum Dots
We propose optical phonon lasing for a double quantum dot (DQD) fabricated in
a semiconductor substrate. We show that the DQD is weakly coupled to only two
LO phonon modes that act as a natural cavity. The lasing occurs for pumping the
DQD via electronic tunneling at rates much higher than the phonon decay rate,
whereas an antibunching of phonon emission is observed in the opposite regime
of slow tunneling. Both effects disappear with an effective thermalization
induced by the Franck-Condon effect in a DQD fabricated in a carbon nanotube
with a strong electron-phonon coupling.Comment: 8 pages, 4 figure
Testing Lorentz Invariance Violation with WMAP Five Year Data
We consider different renormalizable models of Lorentz invariance violation.
We show that the limits on birefringence of the propagation of cosmic microwave
background photons from the five year data of the Wilkinson microwave
anisotropy probe (WMAP) can be translated into a limit of Lorentz symmetry
violation. The obtained limits on Lorentz invariance violation are stronger
than other published limits. We also cast them in terms of limits on a
birefringent effective photo "mass" and on a polarization dependence of the
speed of light.Comment: 5 pages, references and discussions added, results unchanged,
accepted for publications in PR
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