55,954 research outputs found
Competitive function approximation for reinforcement learning
The application of reinforcement learning to problems with continuous domains requires representing the value function by means of function approximation. We identify two aspects of reinforcement learning that make the function approximation process hard: non-stationarity of the target function and biased sampling. Non-stationarity is the result of the bootstrapping nature of dynamic programming where the value function is estimated using its current approximation. Biased sampling occurs when some regions of the state space are visited too often, causing a reiterated updating with similar values which fade out the occasional updates of infrequently sampled regions.
We propose a competitive approach for function approximation where many different local approximators are available at a given input and the one with expectedly best approximation is selected by means of a relevance function. The local nature of the approximators allows their fast adaptation to non-stationary changes and mitigates the biased sampling problem. The coexistence of multiple approximators updated and tried in parallel permits obtaining a good estimation much faster than would be possible with a single approximator. Experiments in different benchmark problems show that the competitive strategy provides a faster and more stable learning than non-competitive approaches.Preprin
Tiling solutions for optimal biological sensing
Biological systems, from cells to organisms, must respond to the ever
changing environment in order to survive and function. This is not a simple
task given the often random nature of the signals they receive, as well as the
intrinsically stochastic, many body and often self-organized nature of the
processes that control their sensing and response and limited resources.
Despite a wide range of scales and functions that can be observed in the living
world, some common principles that govern the behavior of biological systems
emerge. Here I review two examples of very different biological problems:
information transmission in gene regulatory networks and diversity of adaptive
immune receptor repertoires that protect us from pathogens. I discuss the
trade-offs that physical laws impose on these systems and show that the optimal
designs of both immune repertoires and gene regulatory networks display similar
discrete tiling structures. These solutions rely on locally non-overlapping
placements of the responding elements (genes and receptors) that, overall,
cover space nearly uniformly.Comment: 11 page
Evaluation of rate law approximations in bottom-up kinetic models of metabolism.
BackgroundThe mechanistic description of enzyme kinetics in a dynamic model of metabolism requires specifying the numerical values of a large number of kinetic parameters. The parameterization challenge is often addressed through the use of simplifying approximations to form reaction rate laws with reduced numbers of parameters. Whether such simplified models can reproduce dynamic characteristics of the full system is an important question.ResultsIn this work, we compared the local transient response properties of dynamic models constructed using rate laws with varying levels of approximation. These approximate rate laws were: 1) a Michaelis-Menten rate law with measured enzyme parameters, 2) a Michaelis-Menten rate law with approximated parameters, using the convenience kinetics convention, 3) a thermodynamic rate law resulting from a metabolite saturation assumption, and 4) a pure chemical reaction mass action rate law that removes the role of the enzyme from the reaction kinetics. We utilized in vivo data for the human red blood cell to compare the effect of rate law choices against the backdrop of physiological flux and concentration differences. We found that the Michaelis-Menten rate law with measured enzyme parameters yields an excellent approximation of the full system dynamics, while other assumptions cause greater discrepancies in system dynamic behavior. However, iteratively replacing mechanistic rate laws with approximations resulted in a model that retains a high correlation with the true model behavior. Investigating this consistency, we determined that the order of magnitude differences among fluxes and concentrations in the network were greatly influential on the network dynamics. We further identified reaction features such as thermodynamic reversibility, high substrate concentration, and lack of allosteric regulation, which make certain reactions more suitable for rate law approximations.ConclusionsOverall, our work generally supports the use of approximate rate laws when building large scale kinetic models, due to the key role that physiologically meaningful flux and concentration ranges play in determining network dynamics. However, we also showed that detailed mechanistic models show a clear benefit in prediction accuracy when data is available. The work here should help to provide guidance to future kinetic modeling efforts on the choice of rate law and parameterization approaches
Pro-poor intervention strategies in irrigated agriculture in Asia: poverty in irrigated agriculture: issues and options: India
Irrigated farming / Poverty / Institutions / Irrigation programs / Performance evaluation / Irrigation management / Water distribution / Water rates / Cost recovery / India
Dwarf galaxy formation with H2-regulated star formation
We describe cosmological galaxy formation simulations with the adaptive mesh
refinement code Enzo that incorporate a star formation prescription regulated
by the local abundance of molecular hydrogen. We show that this H2-regulated
prescription leads to a suppression of star formation in low mass halos (M_h <
~10^10 M_sun) at z>4, alleviating some of the dwarf galaxy problems faced by
theoretical galaxy formation models. H2 regulation modifies the efficiency of
star formation of cold gas directly, rather than indirectly reducing the cold
gas content with "supernova feedback". We determine the local H2 abundance in
our most refined grid cells (76 proper parsec in size at z=4) by applying the
model of Krumholz, McKee, & Tumlinson, which is based on idealized 1D radiative
transfer calculations of H2 formation-dissociation balance in ~100 pc
atomic--molecular complexes. Our H2-regulated simulations are able to reproduce
the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low
Sigma_gas cutoff due to the transition from atomic to molecular phase and the
metallicity dependence thereof, without the use of an explicit density
threshold in our star formation prescription. We compare the evolution of the
luminosity function, stellar mass density, and star formation rate density from
our simulations to recent observational determinations of the same at z=4-8 and
find reasonable agreement between the two.Comment: replaced with version published in Ap
Data management study, volume 5. Appendix A - Contractor data package technical description and system engineering /SE/ Final report
Technical description and systems engineering contractor data package for Voyager spacecraf
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Pension fund investment and regulation - a macro study
Pension fund assets have been accumulated rapidly during the past decades, and it is
evident that this trend will continue. An immediate problem arising from the rapid
accumulation of such a large volume of assets across countries is how to invest them.
Pension funds differ from other institutional investors, e.g. mutual funds, in that their
investment horizons are relatively long, typically 30-40 years. In addition, they are
pooled assets to support people’s retirement lives. The authorities have a policy
concern about their investment performance, because otherwise, the shortfalls will
have to be met by the nation state (Clark and Hu 2005a). In this paper, we seek to
address this issue from the macro perspective. By using a unique dataset covering 39
countries (17 EMEs and 22 OECD) and based on the classic mean-variance
optimisation approach, first we find a negative impact of international portfolio
investment restrictions on pension fund returns and risk, and this issue is particularly
serious for EMEs. Following a shift from the QAR to the PPR, the average risk is
expected to fall by 27% for EMEs pension funds, while the figure is 10% for OECD
pension funds. Second, there is evidence that if higher portfolio returns are wanted,
higher proportion should be invested in equities and foreign assets. Third, our results
show that pension funds should value the diversification benefit arising from property
investment (Booth 2002)
Media And Government Relations In Papua New Guinea
All is not well with news media in Papua New Guinea. Media and government relations are stressed, a situation adverse to the country's development. Media organisations have to deal with operational difficulties, threats against editorial freedom, and harassment or physical danger experienced by journalists. Yet there are positive factors providing hope for the future, especially that key element, freedom to publish, which goes together with a habit of openess in public life as part of the national culture. That is the main finding of a study made during a working visit to Papua New Guinea
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