On Ptolemaic metric simplicial complexes

We show that under certain mild conditions, a metric simplicial complex which satisfies the Ptolemy inequality is a CAT(0) space. Ptolemy's inequality is closely related to inversions of metric spaces. For a large class of metric simplicial complexes, we characterize those which are isometric to Euclidean space in terms of metric inversions.Comment: 13 page

Natural maps between CAT(0) boundaries

It is shown that certain natural maps between the ideal, Gromov, and end boundaries of a complete CAT(0) space can fail to be either injective or surjective. Additionally the natural map from the Gromov boundary to the end boundary of a complete CAT(-1) space can fail to be either injective or surjective.Comment: 8 page

Housing finance in developing countries : a transaction cost approach

In most developing countries, relatively little mortgage credit is supplied voluntarily, mainly because of the high transaction costs associated with enforcing contracts. In most countries, the supply of mortgage credit is restrained more by the cost of post-contract governance than by the cost of producing contracts. This distinction is important because before-contract costs are dictated by technological conditions - that is, the nature of the production function - so little can be done to change them. The costs of governance, on the other hand, are more amenable to change. If there are significant gains from reducing these costs, institutional reforms may help realize them. In the lowest-income countries, the before-contract transaction costs of providing housing finance are probably high enough per loan dollar that low levels of demand explain the relative smallness of the housing sector. In most other developing countries, housing finance systems could grow more spontaneously and rapidly if there were more effective post-contract enforcement procedures. This growth would improve the efficiency of financial systems and reduce distortions in the economy - so the economic benefits of reducing housing transaction costs are likely to be significant.Banks&Banking Reform,Housing Finance,Public Sector Economics&Finance,Economic Theory&Research,Non Bank Financial Institutions

Rough CAT(0) spaces

We investigate various notions of rough CAT(0). These conditions define classes of spaces that strictly include the union of all Gromov hyperbolic length spaces and all CAT(0) spaces.Comment: Corrected typos and updated Corollaries 3.22 and 3.2

Dynamic mechanical analysis and organization/storage of data for polymetric materials

Dynamic mechanical analysis was performed on a variety of temperature resistant polymers and composite resin matrices. Data on glass transition temperatures and degree of cure attained were derived. In addition a laboratory based computer system was installed and data base set up to allow entry of composite data. The laboratory CPU termed TYCHO is based on a DEC PDP 11/44 CPU with a Datatrieve relational data base. The function of TYCHO is integration of chemical laboratory analytical instrumentation and storage of chemical structures for modeling of new polymeric structures and compound

Deriving the sampling errors of correlograms for general white noise

We derive the second-order sampling properties of certain autocovariance and autocorrelation estimators for sequences of independent and identically distributed samples. Specifically, the estimators we consider are the classic lag windowed correlogram, the correlogram with subtracted sample mean, and the fixed-length summation correlogram. For each correlogram we derive explicit formulas for the bias, covariance, mean square error and consistency for generalised higher-order white noise sequences. In particular, this class of sequences may have non-zero means, be complexed valued and also includes non-analytical noise signals. We find that these commonly used correlograms exhibit lag dependent covariance despite the fact that these processes are white and hence by definition do not depend on lag.Comment: Submitted to Biometrik

Thoughts of Leaving: An Exploration of Why New York City Middle School Teachers Consider Leaving Their Classrooms

This report explores the conditions under which middle-school teachers in New York City leave their schools, and the consequences of this turnover. The focus on middle schools stems from the widely-held view that the middle grades are a critical turning point in the lives of children, and that many New York City school children lose academic momentum in these grades, setting them on trajectories of failure as they move towards high school and life beyond it. This report is based on a survey of more than 4,000 full-time middle school teachers working in 125 of the nearly 200 middle schools in New York City serving children in grades six through eight in the 2009-10 school year. The participating teachers reported whether they had considered leaving their current school or leaving teaching during that school year, and the reasons that they considered leaving. The report links their responses to teachers' reports about their own backgrounds and experiences, to the demographic characteristics of the schools in which they teach, and to the collective perceptions of all of the teachers in a school about that school as a workplace. This report is part of a three-year, mixed-methods study of teacher turnover in New York City middle schools