12,247 research outputs found
The Dreaming Variational Autoencoder for Reinforcement Learning Environments
Reinforcement learning has shown great potential in generalizing over raw
sensory data using only a single neural network for value optimization. There
are several challenges in the current state-of-the-art reinforcement learning
algorithms that prevent them from converging towards the global optima. It is
likely that the solution to these problems lies in short- and long-term
planning, exploration and memory management for reinforcement learning
algorithms. Games are often used to benchmark reinforcement learning algorithms
as they provide a flexible, reproducible, and easy to control environment.
Regardless, few games feature a state-space where results in exploration,
memory, and planning are easily perceived. This paper presents The Dreaming
Variational Autoencoder (DVAE), a neural network based generative modeling
architecture for exploration in environments with sparse feedback. We further
present Deep Maze, a novel and flexible maze engine that challenges DVAE in
partial and fully-observable state-spaces, long-horizon tasks, and
deterministic and stochastic problems. We show initial findings and encourage
further work in reinforcement learning driven by generative exploration.Comment: Best Student Paper Award, Proceedings of the 38th SGAI International
Conference on Artificial Intelligence, Cambridge, UK, 2018, Artificial
Intelligence XXXV, 201
The Structure of Barium in the hcp Phase Under High Pressure
Recent experimental results on two hcp phases of barium under high pressure
show interesting variation of the lattice parameters. They are here interpreted
in terms of electronic structure calculation by using the LMTO method and
generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II
the dramatic drop in c/a is an instability analogous to that in the group II
metals but with the transfer of s to d electrons playing a crucial role in Ba.
Meanwhile in phase V, the instability decrease a lot due to the core repulsion
at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx,
71.15LaComment: 29 pages, 8 figure
Electronic Structure of New LiFeAs High-Tc Superconductor
We present results of it ab initio LDA calculations of electronic structure
of "next generation" layered ironpnictide High-Tc superconductor LiFeAs
(Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently
studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely
close to the Fermi level its electronic properties are also determined ma inly
by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs
are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of
the Fe-3d density o f states and Fermi surface.Comment: 4 pages, 5 figures; Electronic structure improved with respect to new
experimental crystal structure dat
Approximation of corner polyhedra with families of intersection cuts
We study the problem of approximating the corner polyhedron using
intersection cuts derived from families of lattice-free sets in .
In particular, we look at the problem of characterizing families that
approximate the corner polyhedron up to a constant factor, which depends only
on and not the data or dimension of the corner polyhedron. The literature
already contains several results in this direction. In this paper, we use the
maximum number of facets of lattice-free sets in a family as a measure of its
complexity and precisely characterize the level of complexity of a family
required for constant factor approximations. As one of the main results, we
show that, for each natural number , a corner polyhedron with basic
integer variables and an arbitrary number of continuous non-basic variables is
approximated up to a constant factor by intersection cuts from lattice-free
sets with at most facets if and that no such approximation is
possible if . When the approximation factor is allowed to
depend on the denominator of the fractional vertex of the linear relaxation of
the corner polyhedron, we show that the threshold is versus .
The tools introduced for proving such results are of independent interest for
studying intersection cuts
Adequacy of Approximations in GW Theory
We use an all-electron implementation of the GW approximation to analyze
several possible sources of error in the theory and its implementation. Among
these are convergence in the polarization and Green's functions, the dependence
of QP levels on choice of basis sets, and differing approximations for dealing
with core levels. In all GW calculations presented here, G and W are generated
from the local-density approximation (LDA), which we denote as the \GLDA\WLDA
approximation. To test its range of validity, the \GLDA\WLDA approximation is
applied to a variety of materials systems. We show that for simple sp
semiconductors, \GLDA\WLDA always underestimates bandgaps; however, better
agreement with experiment is obtained when the self-energy is not renormalized,
and we propose a justification for it. Some calculations for Si are compared to
pseudopotential-based \GLDA\WLDA calculations, and some aspects of the
suitability of pseudopotentials for GW calculations are discussed.Comment: 38 pages,6 figures. Minor Revision
Momentum-resolved spectral functions of SrVO calculated by LDA+DMFT
LDA+DMFT, the merger of density functional theory in the local density
approximation and dynamical mean-field theory, has been mostly employed to
calculate k-integrated spectra accessible by photoemission spectroscopy. In
this paper, we calculate k-resolved spectral functions by LDA+DMFT. To this
end, we employ the Nth order muffin-tin (NMTO) downfolding to set up an
effective low-energy Hamiltonian with three t_2g orbitals. This downfolded
Hamiltonian is solved by DMFT yielding k-dependent spectra. Our results show
renormalized quasiparticle bands over a broad energy range from -0.7 eV to +0.9
eV with small ``kinks'', discernible in the dispersion below the Fermi energy.Comment: 21 pages, 8 figure
Bidirectional PageRank Estimation: From Average-Case to Worst-Case
We present a new algorithm for estimating the Personalized PageRank (PPR)
between a source and target node on undirected graphs, with sublinear
running-time guarantees over the worst-case choice of source and target nodes.
Our work builds on a recent line of work on bidirectional estimators for PPR,
which obtained sublinear running-time guarantees but in an average-case sense,
for a uniformly random choice of target node. Crucially, we show how the
reversibility of random walks on undirected networks can be exploited to
convert average-case to worst-case guarantees. While past bidirectional methods
combine forward random walks with reverse local pushes, our algorithm combines
forward local pushes with reverse random walks. We also discuss how to modify
our methods to estimate random-walk probabilities for any length distribution,
thereby obtaining fast algorithms for estimating general graph diffusions,
including the heat kernel, on undirected networks.Comment: Workshop on Algorithms and Models for the Web-Graph (WAW) 201
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