Mutualism supports biodiversity when the direct competition is weak

A key question of theoretical ecology is which properties of ecosystems favour their stability and help maintaining biodiversity. This qu estion recently reconsid- ered mutualistic systems, generating intense controversy about the role of mutu- alistic interactions and their network architecture. Here we show analytically and verify with simulations that reducing the effective intersp ecific competition and the propagation of perturbations positively influences struct ural stability against envi- ronmental perturbations, enhancing persistence. Notewor thy, mutualism reduces the effective interspecific competition only when the direct interspecific competition is weaker than a critical value. This critical competition i s in almost all cases larger in pollinator networks than in random networks with the same connectance. Highly connected mutualistic networks reduce the propagation of e nvironmental perturba- tions, a mechanism reminiscent of MacArthur’s proposal tha t ecosystem complexity enhances stability. Our analytic framework rationalizes p revious contradictory re- sults, and it gives valuable insight on the complex relation ship between mutualism and biodiversity

Productivity Differences in OECD Countries

This paper investigates cross-country productivity convergence at a sectoral level using multivariate unit-root tests. Our empirical analysis counts with three distinctive features. First, it allows all the coefficients in the panel specification to vary across countries. Second, it accounts for the presence of significant cross-country correlations found in the data. Third, when the null hypothesis of non convergence is rejected, a second test determines the number of converging countries. Based on a sample of thirteen OECD countries our results show evidence of convergence in three out of six sectors, namely, agriculture, construction, and transportation and communication services.Convergence, panel data, productivity

Enhancement of collagen deposition and cross-linking by coupling lysyl oxidase with bone morphogenetic protein-1 and its application in tissue engineering

Cultured cell-derived extracellular matrices (ECM)-based biomaterials exploit the inherent capacity of cells to create highly sophisticated supramolecular assemblies. However, standard cell culture conditions are far from ideal given the fact that the diluted microenvironment does not favor the production of ECM components, a circumstance particularly relevant for collagen. An incomplete conversion of procollagen by C-proteinase/bone morphogenetic protein 1 (BMP1) has been proposed to severely limit in vitro collagen deposition. BMP1 also catalyzes the proteolytic activation of the precursor of the collagen cross-linking enzyme, lysyl oxidase (LOX) to yield the active form, suggesting a deficit in cross-linking activity under standard conditions. We hypothesized that the implementation of fibroblast cultures with LOX and BMP1 may be an effective way to increase collagen deposition. To test it, we have generated stable cell lines overexpressing LOX and BMP1 and studied the effect of supernatants enriched in LOX and BMP1 on collagen synthesis and deposition from fibroblasts. Herein, we demonstrate that the supplementation with LOX and BMP1 strongly increased the deposition of collagen onto the insoluble matrix at the expense of the soluble fraction in the extracellular medium. Using decellularization protocols, we also show that fibroblast-derived matrices regulate adipogenic and osteogenic differentiation of human mesenchymal stem cells (MSC), and this effect was modulated by LOX/BMP1. Collectively, these data demonstrate that we have developed a convenient protocol to enhance the capacity of in vitro cell cultures to deposit collagen in the ECM, representing this approach a promising technology for application in tissue engineeringTis work was supported by grants from Ministerio de Economía y Competitividad (Plan Nacional de I+D+I: SAF2012-34916, and SAF2015-65679-R to F.R-P

The Greek Crisis: Causes and Consequences

Greece has reached a point where, under any plausible macroeconomic scenario, public debt will continue growing faster than GDP. Fiscal consolidation alone cannot close the solvency gap. A substantial reduction in the stock of debt is needed. Even post-debt restructuring, there is no guarantee that the government will succeed in its dual goal of restoring fiscal solvency and closing the competitiveness gap. Yet we think Greece stands a better chance of accomplishing these goals from inside the EMU rather than outside it. This chapter takes stock of the factors that led to the explosion of public debt, the loss of competitiveness, and the failure of the first EU-IMF programme. We also present our views on the likely debt restructuring (and post-restructuring) scenarios.

Market Structure, Technology Spillovers, and Persistence in Productivity Differentials

Using data from 11 main manufacturing industries in 17 OECD countries, this paper empirically investigates the determinants of cross-country differences in the persistence of productivity differentials Specifically, we focus on the effects of product market structure and technology diffusion. It is found that the manufacturing industries display a wide range of convergence rates. Consistent with theories, the persistence of productivity differentials is found to be positively correlated with the price-cost margin and the intra-industry trade index - the proxies for market monopolistic behavior. The proxies for tecnology diffusion, however, do not exhibit consistently significant effect. Among the conditioning macro variables, productivity convergence appears to be enhanced by human capital but deterred by government spending.Total factor productivity, convergence, market structure, technology diffusion

Testing for Output Convergence: A Re-Examination

This paper investigates output convergence for the G7 countries using multivariate time series techniques. We consider both the null hypotheses of no convergence and convergence. It is shown that inferences on output convergence depend on which one of the two null hypotheses is considered. Further, the no convergence results reported in previous studies using the time series definition may be attributed to the low power of the test procedures being used. Our results also highlight some potential problems on interpreting results from some typical multivariate unit root and stationarity tests.Output convergence, multivariate test, unit root test, stationarity test

Geometric reconstruction and persistence methods

In the present work we reconstruct the homotopy type of an unknown Euclidean subspace from a known sample of data. We carry out such reconstruction through generalized Čech complexes, by choosing radii which are less or equal than the reach of the subspace and by applying the Nerve Lemma. We also approach the reconstruction of a geodesic subspace through its convexity radius and a dense enough sample. Afterwards, we obtain homology and homotopy groups in terms of persistences, together with interleavings and isomorphisms between them. We conclude studying the reconstruction of a particular subspace that has reach equal to zero, where our results cannot be applied.Master's Thesis in MathematicsMAT399MAMN-MA

Low lying excitations of few electrons in quantum dots

[Excerpt from the Introduction]: This thesis focuses on neutral excitations of few-electrons states in nanofabricatedQDsmeasured by inelastic light scattering. The analysis of the spectra demonstrate that this technique can open a venue in the study of quasi-particles at the nanoscale. A model of electronic states confined in QDs is presented in the first chapter, while the second one provides a survey of the relevant experimental studies in this field. Special attention is paid to the process of inelastic light scattering by introducing a description of the selection rules for the scattering processes by electrons in semiconductor QDs. Chapter 3 describes the nano-fabrication techniques of doped AlGaAs/GaAs QDs and the different setups used for the measurements. Experiments performedwith \u3bc-photoluminescence that demonstrate the confinement and the homogeneity of the fabricated QDs are reported at the end of this chapter. Chapter 4 reports inelastic light scattering spectra fromQDs with many electrons. In these experiments the atomic-like shell structure of the electronic states is demonstrated. The framework used to describe this regime of highly populated QDs is similar to the one used for transport experiments in the Coulomb blockade regime.Observations of tunneling excitations in coupled quantumdots with inelastic light scattering will also be presented. The most innovative results of this work are presented in chapter 5 that focuses on the study of interaction phenomena that take place in QDs in the few electron regime. The correlations that occur among the electrons deeply affect the inelastic light spectra in these systems. Themain outcome regards the case of QDs with four electrons where strong correlation effects have been found in the behavior of spin and charge collective excitations. Light scattering spectra from these dots at zero and finite perpendicularmagnetic fieldswill be presented. The evaluation of the excitation spectra as a function of the magnetic field reveals a ground-state transition between correlated states of the four electrons with different spin configurations

Planning fatigue experiments and analyzing fatigue data with the random fatigue-limit model and modified sudden death tests

In this research, we address important issues faced by researchers in fatigue testing. We suggest a practical model to describe the relationship between fatigue life and applied stress, illustrate the corresponding data analysis methods, and study test plans under this model. We also present test plans that provide a systematic and efficient use of a limited number of test positions. These methods emphasize the importance of accuracy in the study of fatigue life while recognizing physical realities and resource limitations;In a fatigue-limit model, test units tested below the fatigue limit (also known as the threshold stress) theoretically will never fail. We use the random fatigue-limit model to describe: (a) the dependence of fatigue life on the stress level, (b) the variation in fatigue life, and (c) the unit-to-unit variation in the fatigue limit. We fit the model to actual fatigue data sets by maximum likelihood methods and study the fits under different distributional assumptions;Using the random fatigue-limit model, we present methods for planning future life tests. We obtain planning values from an actual fatigue experiment and use these to plan future tests. Based on an optimization criterion, we compute best traditional (equally-spaced stress levels with equal allocations of test units) and general (arbitrary choices for stress levels and allocations) plans with 3 and 4 stress levels. The results here serve as guidelines for planning actual tests;We present modified sudden death test (MSDT) plans to address the problem of limited testing positions in single population life tests. A single MSDT involves testing k specimens simultaneously until the rth failure. The complete MSDT plan consists of g single MSDTs run in sequence. We evaluate test plans with respect to the asymptotic variance of maximum likelihood estimators of quantities of interest, total length of the experiment and sample size

Stars and brown dwarfs in the sigma Orionis cluster. III. OSIRIS/GTC low-resolution spectroscopy of variable sources

Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young sigma Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Halpha, and/or X-rays on time scales of hours to years. We complemented our spectra with optical and near-/mid-infrared photometry. Results. For seven targets, we detected lithium in absorption, identified Halpha, the calcium doublet, and forbidden lines in emission, and/or determined spectral types for the first time. We characterise in detail a faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical and a large Halpha equivalent width of -125+/-15 AA, as well as two M1-type, X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission lines, the other with a previously unknown cold disc with a large inner hole. Conclusions. New unrevealed strong accretors and disc hosts, even below the substellar limit, await discovery among the list of known sigma Orionis stars and brown dwarfs that are variable in the optical and have no detailed spectroscopic characterisation yet.Comment: A&A, in press (accepted for publication in section 14. Catalogs and data of Astronomy and Astrophysics