276,184 research outputs found
Restricted Value Iteration: Theory and Algorithms
Value iteration is a popular algorithm for finding near optimal policies for
POMDPs. It is inefficient due to the need to account for the entire belief
space, which necessitates the solution of large numbers of linear programs. In
this paper, we study value iteration restricted to belief subsets. We show
that, together with properly chosen belief subsets, restricted value iteration
yields near-optimal policies and we give a condition for determining whether a
given belief subset would bring about savings in space and time. We also apply
restricted value iteration to two interesting classes of POMDPs, namely
informative POMDPs and near-discernible POMDPs
Towards efficient SimRank computation on large networks
SimRank has been a powerful model for assessing the similarity of pairs of vertices in a graph. It is based on the concept that two vertices are similar if they are referenced by similar vertices. Due to its self-referentiality, fast SimRank computation on large graphs poses significant challenges. The state-of-the-art work [17] exploits partial sums memorization for computing SimRank in O(Kmn) time on a graph with n vertices and m edges, where K is the number of iterations. Partial sums memorizing can reduce repeated calculations by caching part of similarity summations for later reuse. However, we observe that computations among different partial sums may have duplicate redundancy. Besides, for a desired accuracy ϵ, the existing SimRank model requires K = [logC ϵ] iterations [17], where C is a damping factor. Nevertheless, such a geometric rate of convergence is slow in practice if a high accuracy is desirable. In this paper, we address these gaps. (1) We propose an adaptive clustering strategy to eliminate partial sums redundancy (i.e., duplicate computations occurring in partial sums), and devise an efficient algorithm for speeding up the computation of SimRank to 0(Kdn2) time, where d is typically much smaller than the average in-degree of a graph. (2) We also present a new notion of SimRank that is based on a differential equation and can be represented as an exponential sum of transition matrices, as opposed to the geometric sum of the conventional counterpart. This leads to a further speedup in the convergence rate of SimRank iterations. (3) Using real and synthetic data, we empirically verify that our approach of partial sums sharing outperforms the best known algorithm by up to one order of magnitude, and that our revised notion of SimRank further achieves a 5X speedup on large graphs while also fairly preserving the relative order of original SimRank scores
LS 5039 - the counterpart of the unidentified MeV source GRO J1823-12
The COMPTEL experiment on CGRO observed the gamma-ray sky at energies from
0.75 MeV to 30 MeV between April 1991 and June 2000. COMPTEL detected many
gamma-ray sources, among them an unidentified one labeled GRO J1823-12, which
is positionally consistent with the prominent high-mass X-ray binary LS 5039.
Because LS 5039 was established as gamma-ray emitter during recent years, whose
gamma-radiation radiation is modulated along its binary orbit, we reanalysed
the COMPTEL data of GRO J1823-12 including an orbital resolved analysis. We
find a significant MeV source, showing evidence for a modulated MeV flux
corresponding to the orbital period of LS 5039 of about 3.9 days. We show that
its MeV emission is stronger at the orbital part around the inferior conjuction
than at the part of the superior conjunction, being in phase with X-rays and
TeV gamma-rays, however being in anti-phase with GeV gamma-rays. We conclude
that the COMPTEL source GRO J1823-12 is the counterpart of the microquasar
candidate LS 5039, at least for the majority of its MeV emission. The COMPTEL
fluxes, put into multifrequency perspective, provide new constraints on the
modelling of the high-energy emission of LS 5039.Comment: accepted by Astronomy & Astrophysics; 11 pages, 9 figure
Fe/Ni ratio in the Ant Nebula Mz 3
We have analyzed the [Fe II] and [Ni II] emission lines in the bipolar
planetary nebula Mz~3. We find that the [Fe II] and [Ni II] lines arise
exclusively from the central regions. Fluorescence excitation in the formation
process of these lines is negligible for this low-excitation nebula. From the
[Fe II]/[Ni II] ratio, we obtain a higher Fe/Ni abundance ratio with respect to
the solar value. The current result provides further supporting evidence for Mz
3 as a symbiotic Mira.Comment: 2 pages, 1 figure, to be published in the Proceedings of the IAU
Symposium 234: Planetary Nebulae in Our Galaxy and Beyond, eds. M.J. Barlow,
R.H. Mende
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