188,520 research outputs found
A Model Approximation Scheme for Planning in Partially Observable Stochastic Domains
Partially observable Markov decision processes (POMDPs) are a natural model
for planning problems where effects of actions are nondeterministic and the
state of the world is not completely observable. It is difficult to solve
POMDPs exactly. This paper proposes a new approximation scheme. The basic idea
is to transform a POMDP into another one where additional information is
provided by an oracle. The oracle informs the planning agent that the current
state of the world is in a certain region. The transformed POMDP is
consequently said to be region observable. It is easier to solve than the
original POMDP. We propose to solve the transformed POMDP and use its optimal
policy to construct an approximate policy for the original POMDP. By
controlling the amount of additional information that the oracle provides, it
is possible to find a proper tradeoff between computational time and
approximation quality. In terms of algorithmic contributions, we study in
details how to exploit region observability in solving the transformed POMDP.
To facilitate the study, we also propose a new exact algorithm for general
POMDPs. The algorithm is conceptually simple and yet is significantly more
efficient than all previous exact algorithms.Comment: See http://www.jair.org/ for any accompanying file
Onset of Electron Acceleration in a Flare Loop
We carried out detailed analysis of X-ray and radio observations of a simple
flare loop that occurred on 12th August 2002, with the impulsive hard X-ray
(HXR) light curves dominated by a single pulse. The emission spectra of the
early impulsive phase are consistent with an isothermal model in the coronal
loop with a temperature reaching several keVs. A power-law high-energy spectral
tail is evident near the HXR peak time, in accordance with the appearance of
footpoints at high energies, and is well correlated with the radio emission.
The energy content of the thermal component keeps increasing gradually after
the disappearance of this nonthermal component. These results suggest that
electron acceleration only covers a central period of a longer and more gradual
energy dissipation process and that the electron transport within the loop
plays a crucial role in the formation of the inferred power-law electron
distribution. The spectral index of power-law photons shows a very gradual
evolution indicating a quasi-steady state of the electron accelerator, which is
confirmed by radio observations. These results are consistent with the theory
of stochastic electron acceleration from a thermal background. Advanced
modeling with coupled electron acceleration and spatial transport processes is
needed to explain these observations more quantitatively, which may reveal the
dependence of the electron acceleration on the spatial structure of the
acceleration region
Bubbles created from vacuum fluctuation
We show that the bubbles can be created from vacuum
fluctuation in certain De Sitter universe, so the space-time foam-like
structure might really be constructed from bubbles of in the
very early inflating phase of our universe. But whether such foam-like
structure persisted during the later evolution of the universe is a problem
unsolved now.Comment: 6 page
A VLSI architecture of JPEG2000 encoder
Copyright @ 2004 IEEEThis paper proposes a VLSI architecture of JPEG2000 encoder, which functionally consists of two parts: discrete wavelet transform (DWT) and embedded block coding with optimized truncation (EBCOT). For DWT, a spatial combinative lifting algorithm (SCLA)-based scheme with both 5/3 reversible and 9/7 irreversible filters is adopted to reduce 50% and 42% multiplication computations, respectively, compared with the conventional lifting-based implementation (LBI). For EBCOT, a dynamic memory control (DMC) strategy of Tier-1 encoding is adopted to reduce 60% scale of the on-chip wavelet coefficient storage and a subband parallel-processing method is employed to speed up the EBCOT context formation (CF) process; an architecture of Tier-2 encoding is presented to reduce the scale of on-chip bitstream buffering from full-tile size down to three-code-block size and considerably eliminate the iterations of the rate-distortion (RD) truncation.This work was supported in part by the China National High Technologies Research Program (863) under Grant 2002AA1Z142
A Stellar Census of the Tucana-Horologium Moving Group
We report the selection and spectroscopic confirmation of 129 new late-type
(K3-M6) members of the Tuc-Hor moving group, a nearby (~40 pc), young (~40 Myr)
population of comoving stars. We also report observations for 13/17 known
Tuc-Hor members in this spectral type range, and that 62 additional candidates
are likely to be unassociated field stars; the confirmation frequency for new
candidates is therefore 129/191 = 67%. We have used RVs, Halpha emission, and
Li6708 absorption to distinguish contaminants and bona fide members. Our
expanded census of Tuc-Hor increases the known population by a factor of ~3 in
total and by a factor of ~8 for members with SpT>K3, but even so, the K-M dwarf
population of Tuc-Hor is still markedly incomplete. The spatial distribution of
members appears to trace a 2D sheet, with a broad distribution in X and Y, but
a very narrow distribution (+/-5 pc) in Z. The corresponding velocity
distribution is very small, with a scatter of +/-1.1 km/s about the mean UVW
velocity. We also show that the isochronal age (20--30 Myr) and the lithium
depletion age (40 Myr) disagree, following a trend seen in other PMS
populations. The Halpha emission follows a trend of increasing EW with later
SpT, as seen for young clusters. We find that members have been depleted of
lithium for spectral types of K7.0-M4.5. Finally, our purely kinematic and
color-magnitude selection procedure allows us to test the efficiency and
completeness for activity-based selection of young stars. We find that 60% of
K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would be omitted in
Xray selected samples. GALEX UV-selected samples using a previously suggested
criterion for youth achieve completeness of 77% and purity of 78%. We suggest
new selection criteria that yield >95% completeness for ~40 Myr
populations.(Abridged)Comment: Accepted to AJ; 28 pages, 12 figures, 5 tables in emulateapj forma
- …