22,143 research outputs found
Determining WWW User's Next Access and Its Application to Pre-fetching
World-Wide Web (WWW) services have grown to levels where significant delays are expected to happen. Techniques like pre-fetching are likely to help users to personalize their needs, reducing their waiting times. However, pre-fetching is only effective if the right documents are identified and if user's move is correctly predicted. Otherwise, pre-fetching will only waste bandwidth. Therefore, it is productive to determine whether a revisit will occur or not, before starting pre-fetching.
In this paper we develop two user models that help determining user's next move. One model uses Random Walk approximation and the other is based on Digital Signal Processing techniques. We also give hints on how to use such models with a simple pre-fetching technique that we are developing.CNP
Dependence of the Black-body Force on Spacetime Geometry and Topology
In this paper we compute the corrections to the black-body force (BBF)
potential due to spacetime geometry and topology. This recently discovered
attractive force on neutral atoms is caused by the thermal radiation emitted
from black bodies and here we investigate it in relativistic gravitational
systems with spherical and cylindrical symmetries. For some astrophysical
objects we find that the corrected black-body potential is greater than the
flat case, showing that this kind of correction can be quite relevant when
curved spaces are considered. Then we consider four cases: The Schwarzschild
spacetime, the global monopole, the non-relativistic infinity cylinder and the
static cosmic string. For the spherically symmetric case of a massive body, we
find that two corrections appear: One due to the gravitational modification of
the temperature and the other due to the modification of the solid angle
subtended by the atom. We apply the found results to a typical neutron star and
to the Sun. For the global monopole, the modification in the black-body
potential is of topological nature and it is due to the central solid angle
deficit that occurs in the spacetime generated by that object. In the
cylindrical case, which is locally flat, no gravitational correction to the
temperature exists, as in the global monopole case. However, we find the
curious fact that the BBF depends on the topology of the spacetime through the
modification of the azimuthal angle and therefore of the solid angle. For the
static cosmic string we find that the force is null for the zero thickness
case.Comment: 8 pages, 5 figures. Revised versio
Solar type II radio bursts associated with CME expansions as shown by EUV waves
We investigate the physical conditions of the sources of two metric Type-II
bursts associated with CME expansions with the aim of verifying the
relationship between the shocks and the CMEs, comparing the heights of the
radio sources and the heights of the EUV waves associated with the CMEs. The
heights of the EUV waves associated with the events were determined in relation
to the wave fronts. The heights of the shocks were estimated by applying two
different density models to the frequencies of the Type-II emissions and
compared with the heights of the EUV waves. For the 13 June 2010 event, with
band-splitting, the shock speed was estimated from the frequency drifts of the
upper and lower branches of the harmonic lane, taking into account the H/F
frequency ratio fH/fF = 2. Exponential fits on the intensity maxima of the
branches revealed to be more consistent with the morphology of the spectrum of
this event. For the 6 June 2012 event, with no band-splitting and with a clear
fundamental lane on the spectrum, the shock speed was estimated directly from
the frequency drift of the fundamental emission, determined by linear fit on
the intensity maxima of the lane. For each event, the most appropriate density
model was adopted to estimate the physical parameters of the radio source. The
13 June 2010 event presented a shock speed of 664-719 km/s, consistent with the
average speed of the EUV wave fronts of 609 km/s. The 6 June 2012 event was
related to a shock of speed of 211-461 km/s, also consistent with the average
speed of the EUV wave fronts of 418 km/s. For both events, the heights of the
EUV wave revealed to be compatible with the heights of the radio source,
assuming a radial propagation of the shock.Comment: Accepted for publication in Astronomy and Astrophysic
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