Strongly Vertex-Reinforced-Random-Walk on the complete graph

We study Vertex-Reinforced-Random-Walk on the complete graph with weights of the form $w(n)=n^\alpha$, with $\alpha>1$. Unlike for the Edge-Reinforced-Random-Walk, which in this case localizes a.s. on 2 sites, here we observe various phase transitions, and in particular localization on arbitrary large sets is possible, provided $\alpha$ is close enough to 1. Our proof relies on stochastic approximation techniques. At the end of the paper, we also prove a general result ensuring that any strongly reinforced VRRW on any bounded degree graph localizes a.s. on a finite subgraph.Comment: 19 p

Global versus Country-Specific Shocks and International Business Cycles

This paper documents the relative importance of global and country-specific shocks for international business cycles. For this purpose, we rely on a symmetric two-country, dynamic, general-equilibrium model with costly, incomplete, international financial markets. We also relate exogenous technologies and government expenditures to unobservable common and idiosynchratic components, and apply a Kalman filter to extract the associated global and country-specific shocks. We show that the baseline parametrization of the model, including all shocks, closely matches the cyclical fluctuations of key macroeconomic variables for the United States and a non-US aggregate over the post-1975 period. We then experiment alternative parametrizations, isolating the effects of each shock, and find that country-specific technology shocks constitute a prime determinant of international business cycles. Also, global technology shocks have marginal contributions, whereas global and country-specific government-expenditure shocks have negligible effects on cyclical fluctuations.General Equilibrium, Kalman Filter, Symmetric Economies

Efficient dielectric matrix calculations using the Lanczos algorithm for fast many-body G0W0G_0W_0 implementations

We present a $G_0W_0$ implementation that assesses the two major bottlenecks of traditional plane-waves implementations, the summations over conduction states and the inversion of the dielectric matrix, without introducing new approximations in the formalism. The first bottleneck is circumvented by converting the summations into Sternheimer equations. Then, the novel avenue of expressing the dielectric matrix in a Lanczos basis is developed, which reduces the matrix size by orders of magnitude while being computationally efficient. We also develop a model dielectric operator that allows us to further reduce the size of the dielectric matrix without accuracy loss. Furthermore, we develop a scheme that reduces the numerical cost of the contour deformation technique to the level of the lightest plasmon pole model. Finally, the use of the simplified quasi-minimal residual scheme in replacement of the conjugate gradients algorithm allows a direct evaluation of the $G_0W_0$ corrections at the desired real frequencies, without need for analytical continuation. The performance of the resulting $G_0W_0$ implementation is demonstrated by comparison with a traditional plane-waves implementation, which reveals a 500-fold speedup for the silane molecule. Finally, the accuracy of our $G_0W_0$ implementation is demonstrated by comparison with other $G_0W_0$ calculations and experimental results.Comment: 19 pages, 2 figure

Global versus Country-Specific Shocks and International Business Cycles

This paper documents the relative importance of global and country-specific shocks for international business cycles. For this purpose, we rely on a symmetric two-country, dynamic, general-equilibrium model with costly, incomplete, international financial markets. We also relate exogenous technologies and government expenditures to unobservable common and idiosynchratic components, and apply a Kalman filter to extract the associated global and country-specific shocks. We show that the baseline parametrization of the model, including all shocks, closely matches the cyclical fluctuations of key macroeconomic variables for the United States and a non-US aggregate over the post-1975 period. We then experiment alternative parametrizations, isolating the effects of each shock, and find that country-specific technology shocks constitute a prime determinant of international business cycles. Also, global technology shocks have marginal contributions, whereas global and country-specific government-expenditure shocks have negligible effects on cyclical fluctuations.General Equilibrium, Kalman Filter, Symmetric Economies.

A world without farmers? Food production, inclusive development and ecology: Historical Evidences for a New Deal

This paper questions the perspective of a "world without agriculture" which underpins the economic paradigm of "structural transformation" and "modern growth". It does so by recomposing worldwide land and labour productivity trends in caloric terms from 1961 to 2007 and by providing an heuristic model showing that the "Lewis Path" to prosperity is only one out of four possible pathways. It shows that more than half of the world population is rather embarked in a "Lewis Trap" where farmers are increasingly numerous and relatively poorer. It highlights how land scarcity and insufficient job opportunities outside agriculture prevent them to increase their labour productivity and incomes with motorized machineries. The emerging paradigm of "ecological intensification" might contribute to overcome the current deadlocks by redirecting worldwide R&D towards small-scale knowledge-intensive and context-specific agricultures overlapping the manufacture and service sectors. (Résumé d'auteur