1,069,716 research outputs found
Re-Pair Compression of Inverted Lists
Compression of inverted lists with methods that support fast intersection
operations is an active research topic. Most compression schemes rely on
encoding differences between consecutive positions with techniques that favor
small numbers. In this paper we explore a completely different alternative: We
use Re-Pair compression of those differences. While Re-Pair by itself offers
fast decompression at arbitrary positions in main and secondary memory, we
introduce variants that in addition speed up the operations required for
inverted list intersection. We compare the resulting data structures with
several recent proposals under various list intersection algorithms, to
conclude that our Re-Pair variants offer an interesting time/space tradeoff for
this problem, yet further improvements are required for it to improve upon the
state of the art
Contribution of One-Time Pair Correlation Function to Kinetic Phenomena in Nonequilibrium Gas
It has been established in nineteen seventies that in nonequilibrium case the
pair collisions generate non-zero two-particle correlations which are
non-diagonal in momentum space and give the essential contribution to the
current fluctuations of hot electrons. It is shown here that this correlations
give also a contribution to the collision integral, i.e., to kinetic properties
of nonequilibrium gas. The expression for electron energy loss rate P via
phonons is re-derived in detail from this point of view. The contribution of
the non-diagonal part of the nonequilibrium pair correlator to phonon-electron
collision integral and to P is obtained and explicitly calculated in the
electron temperature approximation. It is shown that these results can be
obtained from stochastic non-linear kinetic equation with Langevin fluctuation
force. Such an approach allows to formulate the simple general conditions under
that a contribution of two-particle correlations might be essential in
kinetics. The contribution obtained does not contain the extra powers of small
gas parameter unlike the equilibrium virial decompositions.Comment: 6 pages, based on the report presented at the conference ``Progress
in Nonequilibrium Green's Functions'', Dresden, Germany, 19.-22. August 200
Radiation Reaction Kinetics and Collective QED Signatures
Observing collective effects originating from the interplay between quantum
electrodynamics and plasma physics might be achieved in upcoming experiments.
In particular, the generation of electron-positron pairs and the observation of
their collective dynamics could be simultaneously achieved in a collision
between an intense laser and a highly relativistic electron beam through a
laser frequency shift driven by an increase in the plasma density increase. In
this collision, the radiation of high energy photons will serve a dual purpose:
first, in seeding the cascade of pair generation; and, second, in decelerating
the created pairs for detection. The deceleration results in a detectable shift
in the plasma frequency. This deceleration was previously studied considering
only a small sample of individual pair particles. However, the highly
stochastic nature of the quantum radiation reaction in the strong field regime
limits the descriptive power of the average behavior to the dynamics of pair
particles. Here, we examine the full kinetic evolution of generated pairs in
order to more accurately model the relativistically adjusted plasma density. As
we show, the most effective pair energy for creating observable signatures
occurs at a local minimum, obtained at finite laser field strength due to the
tradeoff between pair deceleration and the relativistic particle oscillation at
increasing laser intensity. For a small number of laser cycles, the quantum
radiation reaction may re-arrange the generated pairs into anisotropic
distributions in momentum space, although, in the one dimensional simulations
considered here, this anisotropy quickly decreases
Exact coherent structures in two-dimensional turbulence identified with convolutional autoencoders
Convolutional autoencoders are used to deconstruct the changing dynamics of
two-dimensional Kolmogorov flow as is increased from weakly chaotic flow
at to a chaotic state dominated by a domain-filling vortex pair at
. The highly accurate embeddings allow us to visualise the evolving
structure of state space and are interpretable using `latent Fourier analysis'
(Page {\em et. al.}, \emph{Phys. Rev. Fluids} \textbf{6}, 2021). Individual
latent Fourier modes decode into vortical structures with a streamwise
lengthscale controlled by the latent wavenumber, , with only a small number
required to accurately represent the flow. Latent Fourier
projections reveal a detached class of bursting events at which merge
with the low-dissipation dynamics as is increased to . We use doubly-
() or triply- () periodic latent Fourier modes to generate guesses
for UPOs (unstable periodic orbits) associated with high-dissipation events.
While the doubly-periodic UPOs are representative of the high-dissipation
dynamics at , the same class of UPOs move away from the attractor at
-- where the associated bursting events typically involve larger-scale
() structure too. At an entirely different embedding structure is
formed within the network in which no distinct representations of small-scale
vortices are observed; instead the network embeds all snapshots based around a
large-scale template for the condensate. We use latent Fourier projections to
find an associated `large-scale' UPO which we believe to be a finite-
continuation of a solution to the Euler equations
A Magnetically-Switched, Rotating Black Hole Model For the Production of Extragalactic Radio Jets and the Fanaroff and Riley Class Division
A model is presented in which both Fanaroff and Riley class I and II
extragalactic jets are produced by magnetized accretion disk coronae in the
ergospheres of rotating black holes. While the jets are produced in the
accretion disk itself, the output power still is an increasing function of the
black hole angular momentum. For high enough spin, the black hole triggers the
magnetic switch, producing highly-relativistic, kinetic-energy-dominated jets
instead of Poynting-flux-dominated ones for lower spin. The coronal mass
densities needed to trigger the switch at the observed FR break power are quite
small (), implying that the source of the jet material
may be either a pair plasma or very tenuous electron-proton corona, not the
main accretion disk itself.
The model explains the differences in morphology and Mach number between FR I
and II sources and the observed trend for massive galaxies to undergo the FR
I/II transition at higher radio power. It also is consistent with the energy
content of extended radio lobes and explains why, because of black hole
spindown, the space density of FR II sources should evolve more rapidly than
that of FR I sources.
If the present model is correct, then the ensemble average speed of
parsec-scale jets in sources distinguished by their FR I morphology (not
luminosity) should be distinctly slower than that for sources with FR II
morphology. The model also suggests the existence of a population of
high-redshift, sub-mJy FR I and II radio sources associated with spiral or
pre-spiral galaxies that flared once when their black holes were formed but
were never again re-kindled by mergers.Comment: 14 pages, 2 figures, final version to appear in Sept Ap
Common vacuum conservation amplitude in the theory of the radiation of mirrors in two-dimensional space-time and of charges in four-dimensional space-time
The action changes (and thus the vacuum conservation amplitudes) in the
proper-time representation are found for an accelerated mirror interacting with
scalar and spinor vacuum fields in 1+1 space. They are shown to coincide to
within the multiplier e^2 with the action changes of electric and scalar
charges accelerated in 3+1 space. This coincidence is attributed to the fact
that the Bose and Fermi pairs emitted by a mirror have the same spins 1 and 0
as do the photons and scalar quanta emitted by charges. It is shown that the
propagation of virtual pairs in 1+1 space can be described by the causal
Green's function \Delta_f(z,\mu) of the wave equation for 3+1 space. This is
because the pairs can have any positive mass and their propagation function is
represented by an integral of the causal propagation function of a massive
particle in 1+1 space over mass which coincides with \Delta_f(z,\mu). In this
integral the lower limit \mu is chosen small, but nonzero, to eliminate the
infrared divergence. It is shown that the real and imaginary parts of the
action change are related by dispersion relations, in which a mass parameter
serves as the dispersion variable. They are a consequence of the same relations
for \Delta_f(z,\mu). Therefore, the appearance of the real part of the action
change is a direct consequence of the causality, according to which real part
of \Delta_f(z,\mu) is nonzero only for timelike and zero intervals.Comment: 23 pages, Latex, revte
- …