Nonparametric identification in nonseparable panel data models with generalized fixed effects

This paper is concerned with extending the familiar notion of fixed effects to nonlinear setups with infinite dimensional unobservables like preferences. The main result is that a generalized version of differencing identifies local average structural derivatives (LASDs) in very general nonseparable models, while allowing for arbitrary dependence between the persistent unobservables and the regressors of interest even if there are only two time periods. These quantities specialize to well known objects like the slope coefficient in the semiparametric panel data binary choice model with fixed effects. We extend the basic framework to include dynamics in the regressors and time trends, and show how distributional effects as well as average effects are identified. In addition, we show how to handle endogeneity in the transitory component. Finally, we adapt our results to the semiparametric binary choice model with correlated coefficients, and establish that average structural marginal probabilities are identified. We conclude this paper by applying the last result to a real world data example. Using the PSID, we analyze the way in which the lending restrictions for mortgages eased between 2000 and 2004.

Strong lensing optical depths in a \LambdaCDM universe

We investigate strong gravitational lensing in the concordance $\Lambda$CDM cosmology by carrying out ray-tracing along past light cones through the Millennium Simulation, the largest simulation of cosmic structure formation ever carried out. We extend previous ray-tracing methods in order to take full advantage of the large volume and the excellent spatial and mass resolution of the simulation. As a function of source redshift we evaluate the probability that an image will be highly magnified, will be highly elongated or will be one of a set of multiple images. We show that such strong lensing events can almost always be traced to a single dominant lensing object and we study the mass and redshift distribution of these primary lenses. We fit analytic models to the simulated dark halos in order to study how our optical depth measurements are affected by the limited resolution of the simulation and of the lensing planes that we construct from it. We conclude that such effects lead us to underestimate total strong-lensing cross sections by about 15 percent. This is smaller than the effects expected from our neglect of the baryonic components of galaxies. Finally we investigate whether strong lensing is enhanced by material in front of or behind the primary lens. Although strong lensing lines-of-sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few percent of the total surface density.Comment: version accepted for publicatio

A two-channel Kondo impurity in the spin-1/2 chain: Consequences for Knight shift experiments

A magnetic impurity in the spin-1/2 chain is a simple realization of the two-channel Kondo problem since the field theoretical descriptions in the spin-sector are identical. The correlation functions near the impurity can be calculated. Using a modified version of the numerical transfer matrix DMRG, we are able to accurately determine local properties close to the impurity in the thermodynamic limit. The local susceptibilities (Knight-shifts) show an interesting behavior in a large range around the impurities. We are able to make quantitative experimental predictions which would allow to observe two-channel Kondo physics for the first time directly by doping of spin-1/2 chain compounds.Comment: 2 pages in revtex format including 2 embedded figures (using epsf

The “treatment gap” in global mental health reconsidered: sociotherapy for collective trauma in Rwanda

Background: The “treatment gap” (TG) for mental disorders refers to the difference that exists between the number of people who need care and those who receive care. The concept is strongly promoted by the World Health Organization and widely used in the context of low- and middle-income countries. Although accepting the many demonstrable benefits that flow from this approach, it is important to critically reflect on the limitations of the concept of the TG and its implications for building capacity for mental health services in Rwanda. Objective: The article highlights concerns that the evidence base for mental health interventions is not globally valid, and problematizes the preponderance of psychiatric approaches in international guidelines for mental health. Specifically, the risk of medicalization of social problems and the limited way in which “community” has been conceptualized in global mental health discourses are addressed. Rather than being used as a method for increasing economic efficiency (i.e., reducing healthcare costs), “community” should be promoted as a means of harnessing collective strengths and resources to help promote mental well-being. This may be particularly beneficial for contexts, like Rwanda, where community life has been disrupted by collective violence, and the resulting social isolation constitutes an important determinant of mental distress. Conclusions: Moving forward there is a need to consider alternative paradigms where individual distress is understood as a symptom of social distress, which extends beyond the more individually oriented TG paradigm. Sociotherapy, an intervention used in Rwanda over the past 10 years, is presented as an example of how communities of support can be built to promote mental health and psychosocial well-being

Uniform approximation of homeomorphisms by diffeomorphisms

We prove that a compactly supported homeomorphism of a smooth manifold of dimension greater or equal to 5 can be approximated uniformly by compactly supported diffeomorphisms if and only if it is isotopic to a diffeomorphism. If the given homeomorphism is in addition volume preserving, then it can be approximated uniformly by volume preserving diffeomorphisms.Comment: v4: 5 pages; long overdue revision; clarified and improved statements, notation, and proofs. The main theorem may already be known, but I have not been able to find a precise reference. After talking to a number of topologists without getting a satisfactory answer, I decided to write up the proof myself. Comments and references welcom

Evidence for Orbital Order and its Relation to Superconductivity in FeSe0.4Te0.6

The emergence of nematic electronic states accompanied by a structural phase transition is a recurring theme in many correlated electron materials, including the high-temperature copper oxide- and iron-based superconductors. We provide evidence for nematic electronic states in the iron-chalcogenide superconductor FeSe0.4Te0.6 from quasi-particle scattering detected in spectroscopic maps. The symmetry-breaking states persist above Tc into the normal state. We interpret the scattering patterns by comparison with quasi-particle interference patterns obtained from a tight-binding model, accounting for orbital ordering. The relation to superconductivity and the influence on the coherence length are discussed.Comment: 5 pages, 5 figures, updated with published versio

Abundances, masses, and weak-lensing mass profiles of galaxy clusters as a function of richness and luminosity in LambdaCDM cosmologies

We test the concordance LCDM cosmology by comparing predictions for the mean properties of galaxy clusters to observations. We use high-resolution N-body simulations of cosmic structure formation and semi-analytic models of galaxy formation to compute the abundance, mean density profile, and mass of galaxy clusters as a function of richness and luminosity, and we compare these predictions to observations of clusters in the Sloan Digital Sky Survey (SDSS) maxBCG catalogue. We discuss the scatter in the mass-richness relation, the reconstruction of the cluster mass function from the mass-richness relation, and fits to the weak-lensing cluster mass profiles. The impact of cosmological parameters on the predictions is investigated by comparing results from galaxy models based on the Millennium Simulation (MS) and another WMAP1 simulation to those from a WMAP3 simulation. We find that the simulated weak-lensing mass profiles and the observed profiles of the SDSS maxBCG clusters agree well in shape and amplitude. The mass-richness relations in the simulations are close to the observed relation, with differences lesssim 30%. The MS and WMAP1 simulations yield cluster abundances similar to those observed, whereas abundances in the WMAP3 simulation are 2-3 times lower. The differences in cluster abundance, mass, and density amplitude between the simulations and the observations can be attributed to differences in the underlying cosmological parameters, in particular the power spectrum normalisation sigma_8. Better agreement between predictions and observations should be reached with a normalisation $0.722<sigma_8<0.9$ (probably closer to the upper value), i.e. between the values underlying the two simulation sets.Comment: revised version taking reviewer's comments into account, submitted to MNRA