The Bianchi groups are subgroup separable on geometrically finite subgroups

We show that for certain arithmetic groups, geometrically finite subgroups are the intersection of finite index subgroups containing them. Examples are the Bianchi groups and the Seifert-Weber dodecahedral space. In particular, for manifolds commensurable with these groups, immersed incompressible surfaces lift to embeddings in a finite sheeted covering.Comment: 19 page

All flat manifolds are cusps of hyperbolic orbifolds

We show that all closed flat n-manifolds are diffeomorphic to a cusp cross-section in a finite volume hyperbolic (n+1)-orbifold.Comment: Published by Algebraic and Geometric Topology at http://www.maths.warwick.ac.uk/agt/AGTVol2/agt-2-13.abs.htm

On the geometric boundaries of hyperbolic 4-manifolds

We provide, for hyperbolic and flat 3-manifolds, obstructions to bounding hyperbolic 4-manifolds, thus resolving in the negative a question of Farrell and Zdravkovska.Comment: 8 pages. Published copy, also available at http://www.maths.warwick.ac.uk/gt/GTVol4/paper5.abs.htm

First-Year Spectroscopy for the SDSS-II Supernova Survey

This paper presents spectroscopy of supernovae discovered in the first season of the Sloan Digital Sky Survey-II Supernova Survey. This program searches for and measures multi-band light curves of supernovae in the redshift range z = 0.05 - 0.4, complementing existing surveys at lower and higher redshifts. Our goal is to better characterize the supernova population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators and as probes of dark energy. Our supernova spectroscopy program features rapid-response observations using telescopes of a range of apertures, and provides confirmation of the supernova and host-galaxy types as well as precise redshifts. We describe here the target identification and prioritization, data reduction, redshift measurement, and classification of 129 SNe Ia, 16 spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first season. We also describe our efforts to measure and remove the substantial host galaxy contamination existing in the majority of our SN spectra.Comment: Accepted for publication in The Astronomical Journal(47pages, 9 figures

Patient-Reported Outcomes Measurement Information System in Children with Crohn's Disease

To assess the criterion validity and responsiveness of Patient Reported Outcomes Measurement Information System (PROMIS) in a web-based cohort of children with Crohn’s disease

What Do Unions Do for Economic Performance?

Twenty years have passed since Freeman and Medoff's What Do Unions Do? This essay assesses their analysis of how unions in the U.S. private sector affect economic performance - productivity, profitability, investment, and growth. Freeman and Medoff are clearly correct that union productivity effects vary substantially across workplaces. Their conclusion that union effects are on average positive and substantial cannot be sustained, subsequent evidence suggesting an average union productivity effect near zero. Their speculation that productivity effects are larger in more competitive environments appears to hold up, although more evidence is needed. Subsequent literature continues to find unions associated with lower profitability, as noted by Freeman and Medoff. Unions are found to tax returns stemming from market power, but industry concentration is not the source of such returns. Rather, unions capture firm quasi-rents arising from long-lived tangible and intangible capital and from firm-specific advantages. Lower profits and the union tax on asset returns leads to reduced investment and, subsequently, lower employment and productivity growth. There is little evidence that unionization leads to higher rates of business failure. Given the decline in U.S. private sector unionism, I explore avenues through which individual and collective voice might be enhanced, focusing on labor law and workplace governance defaults. Substantial enhancement of voice requires change in the nonunion sector and employer as well as worker initiatives. It is unclear whether labor unions would be revitalized or further marginalized by such an evolution