6,029 research outputs found
Scalable First-Order Methods for Robust MDPs
Robust Markov Decision Processes (MDPs) are a powerful framework for modeling
sequential decision-making problems with model uncertainty. This paper proposes
the first first-order framework for solving robust MDPs. Our algorithm
interleaves primal-dual first-order updates with approximate Value Iteration
updates. By carefully controlling the tradeoff between the accuracy and cost of
Value Iteration updates, we achieve an ergodic convergence rate of for the best
choice of parameters on ellipsoidal and Kullback-Leibler -rectangular
uncertainty sets, where and is the number of states and actions,
respectively. Our dependence on the number of states and actions is
significantly better (by a factor of ) than that of pure
Value Iteration algorithms. In numerical experiments on ellipsoidal uncertainty
sets we show that our algorithm is significantly more scalable than
state-of-the-art approaches. Our framework is also the first one to solve
robust MDPs with -rectangular KL uncertainty sets
Using factor models to construct composite indicators from BCS data - a comparison with European Commission confidence indicators
This paper compares different approaches to constructing composite business cycle indicators based on series from the Joint Harmonised EU Programme of Business and Consumer Surveys (BCS). The currently computed Confidence Indicators are used as benchmarks in gauging four different factor-analytic methods to extract sectoral business cycle indicators. Improvements in tracking performance are mainly found on individual country level.Business cycle, Confidence indicators, Factor models, Principal components, Business and Consumer Surveys (BCS), Gayer, Genet
Total and selective reuse of Krylov subspaces for the resolution of sequences of nonlinear structural problems
This paper deals with the definition and optimization of augmentation spaces
for faster convergence of the conjugate gradient method in the resolution of
sequences of linear systems. Using advanced convergence results from the
literature, we present a procedure based on a selection of relevant
approximations of the eigenspaces for extracting, selecting and reusing
information from the Krylov subspaces generated by previous solutions in order
to accelerate the current iteration. Assessments of the method are proposed in
the cases of both linear and nonlinear structural problems.Comment: International Journal for Numerical Methods in Engineering (2013) 24
page
Probing non-standard gravity with the growth index: a background independent analysis
Measurements of the growth index provide a clue as to whether
Einstein's field equations encompass gravity also on large cosmic scales, those
where the expansion of the universe accelerates. We show that the information
encoded in this function can be satisfactorily parameterized using a small set
of coefficients in such a way that the true scaling of the growth
index is recovered to better than in most dark energy and dark gravity
models. We find that the likelihood of current data is maximal for
and , a measurement compatible
with the CDM predictions. Moreover data favor models predicting
slightly less growth of structures than the Planck LambdaCDM scenario. The main
aim of the paper is to provide a prescription for routinely calculating, in an
analytic way, the amplitude of the growth indices in relevant
cosmological scenarios, and to show that these parameters naturally define a
space where predictions of alternative theories of gravity can be compared
against growth data in a manner which is independent from the expansion history
of the cosmological background. As the standard -plane provides a tool
to identify different expansion histories and their relation to various
cosmological models, the -plane can thus be used to locate different
growth rate histories and their relation to alternatives model of
gravity. As a result, we find that the Dvali-Gabadadze-Porrati gravity model is
rejected with a confidence level. By simulating future data sets, such
as those that a Euclid-like mission will provide, we also show how to tell
apart LambdaCDM predictions from those of more extreme possibilities, such as
smooth dark energy models, clustering quintessence or parameterized
post-Friedmann cosmological models.Comment: 29 pages, 21 figure
On the role of the Jeffreys'sheltering mechanism in the sustain of extreme water waves
The effect of the wind on the sustain of extreme water waves is investigated
experimentally and numerically. A series of experiments conducted in the Large
Air-Sea Interactions Facility (LASIF) showed that a wind blowing over a
strongly nonlinear short wave group due to the linear focusing of a modulated
wave train may increase the life time of the extreme wave event. The expriments
suggested that the air flow separation that occurs on the leeward side of the
steep crests may sustain longer the maximum of modulation of the
focusing-defocusing cycle. Based on a Boundary-Integral Equation Method and a
pressure distribution over the steep crests given by the Jeffreys'sheltering
theory, similar numerical simulations have confirmed the experimental resultsComment: accept\'{e} pour publication 200
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