Multi-objective optimization of power electronic converters

L'abstract è presente nell'allegato / the abstract is in the attachmen

Situation and challenges of the Mexicana organic sector

For Mexico, organic farming is a feasible option, given the goodness it offers the country, since it is linked to the geography of poverty, mainly with small-scale farmers with surfaces below 3 ha, 22 indigenous groups, and 35% of women responsible for farming. In addition, it is linked to a form of production that protects and preserves natural resources

Modeling Adoption and Usage of Competing Products

The emergence and wide-spread use of online social networks has led to a dramatic increase on the availability of social activity data. Importantly, this data can be exploited to investigate, at a microscopic level, some of the problems that have captured the attention of economists, marketers and sociologists for decades, such as, e.g., product adoption, usage and competition. In this paper, we propose a continuous-time probabilistic model, based on temporal point processes, for the adoption and frequency of use of competing products, where the frequency of use of one product can be modulated by those of others. This model allows us to efficiently simulate the adoption and recurrent usages of competing products, and generate traces in which we can easily recognize the effect of social influence, recency and competition. We then develop an inference method to efficiently fit the model parameters by solving a convex program. The problem decouples into a collection of smaller subproblems, thus scaling easily to networks with hundred of thousands of nodes. We validate our model over synthetic and real diffusion data gathered from Twitter, and show that the proposed model does not only provides a good fit to the data and more accurate predictions than alternatives but also provides interpretable model parameters, which allow us to gain insights into some of the factors driving product adoption and frequency of use

Explaining the dollar/euro exchange rate: the role of policy uncertainty, asymmetric information, and hedging opportunities

Many observers were surprised by the depreciation of the euro after its launch in 1999. Handicapped by a short sample, explanations tended to appeal to anecdotes and lessons learned from the experiences of other currencies. Now sample sizes are just becoming large enough to permit reasonable empirical analyses. This paper begins with a theoretical model addressing transaction costs of trading the euro. The model of pre- and post-euro foreign exchange trading explains wider spreads on the euro as a result of three possible causes: a reduction in hedging opportunities due to the elimination of the legacy currencies, policy uncertainty on the part of the ECB, and asymmetric information due to some traders having prior knowledge of ECB policies. However, even informal empirical evidence tends to reject the hypothesis that spreads were larger on the euro than the mark for all but the first few months. This seems like an unlikely candidate to explain euro depreciation over the prolonged period observed. After addressing spreads, the model is turned toward an explanation of the exchange rate level. By specializing the fundamentals considered to the euro-area inflation rate, the model is structured toward the dynamics of learning about ECB policy with regard to inflation. While a stated target inflation rate of 2 percent existed, it may be that market participants had to be convinced that the ECB would, indeed, generate low and stable inflation. The theory motivates an empirical model of Bayesian updating related to market participants learning about the underlying inflation process under the ECB regime. With a prior distribution drawn from the pre-euro EMS experience and updating based upon the realized experience each month following the introduction of the euro, the evidence suggests that it was not until the fall of 2000 that the market assessed a greater than 50 percent probability that the inflation process had changed to a new regime. From this point on, trend depreciation of the euro ends and further increases in the probability of the new inflation process are associated with euro appreciation.euro, foreign exchange, Bayesian learning

Submodular Inference of Diffusion Networks from Multiple Trees

Diffusion and propagation of information, influence and diseases take place over increasingly larger networks. We observe when a node copies information, makes a decision or becomes infected but networks are often hidden or unobserved. Since networks are highly dynamic, changing and growing rapidly, we only observe a relatively small set of cascades before a network changes significantly. Scalable network inference based on a small cascade set is then necessary for understanding the rapidly evolving dynamics that govern diffusion. In this article, we develop a scalable approximation algorithm with provable near-optimal performance based on submodular maximization which achieves a high accuracy in such scenario, solving an open problem first introduced by Gomez-Rodriguez et al (2010). Experiments on synthetic and real diffusion data show that our algorithm in practice achieves an optimal trade-off between accuracy and running time.Comment: To appear in the 29th International Conference on Machine Learning (ICML), 2012. Website: http://www.stanford.edu/~manuelgr/network-inference-multitree

Trade Liberalization and Regional Income Convergence in Mexico: a Time-Series Analysis

We study the hypothesis of convergence amongst Mexican regions since 1940 with special interest in the post-trade liberalization period. A standard time-series convergence test shows that per capita income levels between the capital and the rest of the regions tend to narrow over time. Using the concept of deterministic and stochastic convergence, we describe the specific characteristics of the growth pattern for each of the regions. We find evidence that supports the hypothesis that trade reforms reversed the convergence process of some regions, especially those less developed. Results further suggest that trade liberalization did not contribute to per capita income convergence between the U.S. and Mexico border regions.Catching-up, Convergence, Deterministic Trend, Unit Root

Income Convergence: The Dickey-Fuller Test under the Simultaneous Presence of Stochastic and Deterministic Trends

We investigate the efficiency of the Dickey-Fuller (DF) test as a tool to examine the convergence hypothesis. In doing so, we first describe two possible outcomes, overlooked in previous studies, namely Loose Catching-up and Loose Lagging-behind. Results suggest that this test is useful when the intention is to discriminate between a unit root process and a trend stationary process, though unreliable when used to differentiate between a unit root process and a process with both deterministic and stochastic trends. This issue may explain the lack of support for the convergence hypothesis in the aforementioned literature.Divergence, Loose Catching-up/Lagging-behind, Convergence, Deterministic and Stochastic trends

Inflation and breaks: the validity of the Dickey-Fuller test

This article proves the asymptotic efficiency of the Dickey Fuller (DF) test when the Data Generating Process of the variable under consideration is in fact mean stationary with breaks. Monte Carlo simulations show that asymptotic properties remain valid for sample sizes of practical interest. We illustrate its performance by studying inflation rate series, a variable that should be stationary if the monetary authority follows an effective inflation targeting regime: shocks are short-lived, therefore, inflation fluctuates randomly around pre-specified targets.Dickey-Fuller test, Mean Stationary Process, Structural Breaks

Testing for a Deterministic Trend when there is Evidence of Unit-Root

Whilst the existence of a unit root implies that current shocks have permanent effects, in the long run, the simultaneous presence of a deterministic trend obliterates that consequence. As such, the long-run level of macroeconomic series depends upon the existence of a deterministic trend. This paper proposes a formal statistical procedure to distinguish between the null hypothesis of unit root and that of unit root with drift. Our procedure is asymptotically robust with regard to autocorrelation and takes into account a potential single structural break. Empirical results show that most of the macroeconomic time series originally analyzed by Nelson and Plosser (1982) are characterized by their containing both a deterministic and a stochastic trend.Unit Root, Deterministic Trend, Trend Regression, R2