Multi-objective reinforcement learning with continuous pareto frontier approximation

This paper is about learning a continuous approximation of the Pareto frontier in Multi-Objective Markov Decision Problems (MOMDPs). We propose a policy-based approach that exploits gradient information to generate solutions close to the Pareto ones. Differently from previous policy-gradient multi-objective algorithms, where n optimization routines are used to have n solutions, our approach performs a single gradient-ascent run that at each step generates an improved continuous approximation of the Pareto frontier. The idea is to exploit a gradient-based approach to optimize the parameters of a function that defines a manifold in the policy parameter space so that the corresponding image in the objective space gets as close as possible to the Pareto frontier. Besides deriving how to compute and estimate such gradient, we will also discuss the non-trivial issue of defining a metric to assess the quality of the candidate Pareto frontiers. Finally, the properties of the proposed approach are empirically evaluated on two interesting MOMDPs

Modelling the potential distribution of Bemisia tabaci in Europe in light of the climate change scenario

BACKGROUND:Bemisia tabaci is a serious pest of agriculturalandhorticultural crops ingreenhousesandfields aroundtheworld. This paper deals with the distribution of the pest under field conditions. In Europe, the insect is currently found in coastal regions of Mediterranean countries where it is subject to quarantine regulations. To assess the risk presented by B. tabaci to Europe, the area of potential establishment of this insect, in light of the climate change scenario,was assessed by a temperature-dependent physiologically based demographicmodel (PBDM). RESULTS: The simulated potential distribution under current climate conditions has been successfully validated with the available field records of B. tabaci in Europe. Considering climate change scenarios of+1and+2 ∘C, range expansion by B. tabaci is predicted, particularly in Spain, France, Italy, Greece and along the Adriatic coast of the Balkans. Nonetheless, even under the scenario of +2 ∘C, northern European countries are not likely to be at risk of B. tabaci establishment because of climatic limitations. CONCLUSION: Model validation with field observations and evaluation of uncertainties associated with model parameter variability support the reliability of model results. The PBDM developed here can be applied to other organisms and offers significant advantages for assessing the potential distribution of invasive species

Fiscal Reforms during Fiscal Consolidation: The Case of Italy

In this paper we aim to discuss the strengths and weaknesses of the fiscal consolidation package adopted recently by the Italian Government in order to achieve a balanced budget by 2013. Revenues are forecasted to increase by more than 3.3 GDP percentage points; these stem mostly from indirect and property taxation. The analysis of the Italian case is interesting since it seems to be consistent with a recent strand of the literature which, in order to foster both short and long-term economic growth, advocated a shift of the tax burden from capital and labour income to consumption and property. Through a set of micro simulation models, this paper evaluates the effects of the Italian fiscal package on households and firms. We show that, in respect of households’ income, indirect and property tax reforms are highly regressive, whilst the reform makes limited resources available for growth enhancing policies (reduction in the effective corporate tax burden). Then, we propose an alternative fiscal package. We show that a less regressive reform on households can be obtained by shifting taxation from personal and corporate income tax to indirect taxation. Our proposal allows the tax burden on firms to be reduced substantially and, in the meantime, offers lower personal income tax rates on households in the lowest deciles of income distribution since they are penalized most by the increase in indirect taxation.tax reforms, fiscal consolidation, micro simulation models, Italy

Fiscal Reforms during Fiscal Consolidation: The Case of Italy

In this paper we aim to discuss the strengths and weaknesses of the fiscal consolidation package adopted recently by the Italian Government in order to achieve a balanced budget by 2013. Revenues are forecasted to increase by more than 3.3 GDP percentage points; these stem mostly from indirect and property taxation. The analysis of the Italian case is interesting since it seems to be consistent with a recent strand of the literature which, in order to foster both short and long-term economic growth, advocated a shift of the tax burden from capital and labour income to consumption and property. Through a set of micro simulation models, this paper evaluates the effects of the Italian fiscal package on households and firms. We show that, in respect of households’ income, indirect and property tax reforms are highly regressive, whilst the reform makes limited resources available for growth enhancing policies (reduction in the effective corporate tax burden). Then, we propose an alternative fiscal package. We show that a less regressive reform on households can be obtained by shifting taxation from personal and corporate income tax to indirect taxation. Our proposal allows the tax burden on firms to be reduced substantially and, in the meantime, offers lower personal income tax rates on households in the lowest deciles of income distribution since they are penalized most by the increase in indirect taxation.Tax reforms, Fiscal consolidation, Micro simulation models, Italy

Low Energy Excitations in Spin Glasses from Exact Ground States

We investigate the nature of the low-energy, large-scale excitations in the three-dimensional Edwards-Anderson Ising spin glass with Gaussian couplings and free boundary conditions, by studying the response of the ground state to a coupling-dependent perturbation introduced previously. The ground states are determined exactly for system sizes up to 12^3 spins using a branch and cut algorithm. The data are consistent with a picture where the surface of the excitations is not space-filling, such as the droplet or the ``TNT'' picture, with only minimal corrections to scaling. When allowing for very large corrections to scaling, the data are also consistent with a picture with space-filling surfaces, such as replica symmetry breaking. The energy of the excitations scales with their size with a small exponent \theta', which is compatible with zero if we allow moderate corrections to scaling. We compare the results with data for periodic boundary conditions obtained with a genetic algorithm, and discuss the effects of different boundary conditions on corrections to scaling. Finally, we analyze the performance of our branch and cut algorithm, finding that it is correlated with the existence of large-scale,low-energy excitations.Comment: 18 Revtex pages, 16 eps figures. Text significantly expanded with more discussion of the numerical data. Fig.11 adde

A new method for analyzing ground-state landscapes: ballistic search

A ``ballistic-search'' algorithm is presented which allows the identification of clusters (or funnels) of ground states in Ising spin glasses even for moderate system sizes. The clusters are defined to be sets of states, which are connected in state-space by chains of zero-energy flips of spins. The technique can also be used to estimate the sizes of such clusters. The performance of the method is tested with respect to different system sizes and choices of parameters. As an application the ground-state funnel structure of two-dimensional +or- J spin glasses of systems up to size L=20 is analyzed by calculating a huge number of ground states per realization. A T=0 entropy per spin of s_0=0.086(4)k_B is obtained.Comment: 10 pages, 11 figures, 35 references, revte

Modeling and simulation of a centrifugal compressor for automotive fuel cell applications

LAUREA MAGISTRALELo scopo del presente lavoro è quello di sviluppare un codice per la modellazione di un compressore centrifugo, applicandolo al design di una macchina per uso con celle a combustibile in ambito automobilistico. Il modello sviluppato si basa su un approccio a linea media, usando correlazioni sperimentali calibrate su diverse geometrie esistenti. Le principali caratteristiche del modello sono la possibilità di implementare una geometria variabile, e la possibilità di lavorare con un modello di fluido generico. Considerato che il fluido usato dalla macchina è aria umida, due modelli sono stati implementati per il fluido: il primo usa l’ipotesi di gas perfetto, mentre il secondo è basato sul modello REFPROP di una miscela generica, con la possibilità di considerare anche la regione bifase. La possibilità di valutare rapidamente le prestazioni di una data geometria è stata usata per impostare una routine di ottimizzazione multivariabile e multi-punto, ottenendo una macchina con rendimento elevato su una vasta gamma di condizioni operative. La mappa delle prestazioni del compressore è stata costruita, e poi interpolata per permetterne l’uso nelle simulazioni. Un modello CAD 3D della macchina è stato successivamente realizzato per valutare la fattibilità meccanica del design. Infine, un modello dinamico e logica di controllo sono stati implementati, per essere usati in simulazioni dell’intero veicolo. La logica di controllo include un controllore in retroazione, una compensazione feed-forward del carico, e un dispositivo anti-pompaggio. Diverse simulazioni hanno confermato la possibilità di ottenere una buona risposta dinamica.The purpose of this work is to develop a tool for the modeling of a centrifugal compressor, using it to design a machine for automotive fuel cell applications. The model developed is based on a mean line approach, using experimental loss correlations calibrated against several existing geometries. The main features of the model are the possibility to implement variable geometry, and the possibility to work with a generic fluid model. Considering that the fluid used in the machine is humid air, two fluid models were implemented: the first uses the perfect gas hypothesis, while the second is based on the REFPROP model of a generic mixture, with the capability to consider also the two-phase region. The possibility to rapidly evaluate the performances of a given geometry was used to set up a multi-variable, multi-point optimization routine, obtaining a machine with high efficiency over a vast range of operating conditions. The performance map of the compressor was built, and then interpolated in order to be used in simulations. A 3D CAD model of the machine was then built to assess the mechanical feasibility of the design. Finally, a dynamic model and control logic were implemented to be used in simulations of the entire vehicle. The control logic includes a feedback controller, a feed-forward load compensation, and an anti-surge device. Different simulations confirmed the possibility to obtain a good dynamic response