Independence and Alpern Multitowers

Let $T$ be any invertible, ergodic, aperiodic measure-preserving transformation of a Lebesgue probability space (X, \calB, \mu), and \P\, any finite measurable partition of $X$. We show that a (finite) Alpern multitower may always be constructed whose base is independent of \P

Xwnt-5A: a maternal Wnt that affects morphogenetic movements after overexpression in embryos of Xenopus laevis

To contribute to an understanding of the roles and mechanisms of action of Wnts in early vertebrate development, we have characterized the normal expression of Xenopus laevis Wnt-5A, and investigated the consequences of misexpression of this putative signalling factor. Xwnt-5A transcripts are expressed throughout development, and are enriched in both the anterior and posterior regions of embryos at late stages of development, where they are found primarily in ectoderm, with lower levels of expression in mesoderm. Overexpression of Xwnt-5A in Xenopus embryos leads to complex malformations distinct from those achieved by ectopic expression of Xwnts −1, −3A, or −8. This phenotype is unlikely to result from Xwnt-5A acting as an inducing agent, as overexpression of Xwnt-5A does not rescue dorsal structures in UV-irradiated embryos, does not induce mesoderm in blastula caps, and Xwnt-5A does not alter the endogenous patterns of expression of goosecoid, Xbra, or Xwnt-8. To pursue whether Xwnt-5A has the capacity to affect morphogenetic movements, we investigated whether overexpression of Xwnt-5A alters the normal elongation of blastula cap explants induced by activin. Intriguingly, Xwnt-5A blocks the elongation of blastula caps in response to activin, without blocking the differentiation of either dorsal or ventral mesoderm within these explants. The data are consistent with Xwnt-5A having the potential activity of modifying the morphogenetic movements of tissues

D-Brane Recoil and Supersymmetry Obstruction

We discuss a model in which our universe is pictured as a recoiling Dirichlet brane: we find that a proper treatment of the recoil leads naturally to supersymmetry obstruction on the four-dimensional world. An essential feature of our approach is the fact that the underlying worldsheet sigma model is non-critical, and the Liouville mode plays the role of the target time. Also, the extra bulk dimensions are viewed as sigma model couplings, and as such have to be averaged by appropriate summation over worldsheet genera. The recoiling brane is in an excited state rather than its ground state, to which it relaxes asymptotically in time, restoring supersymmetry. We also find that the excitation energy, which is considered as the observable effective cosmological `constant' on the brane, is naturally small and can accommodate upper bounds from observations.Comment: 9 pages, no figure

The Bulk RS KK-gluon at the LHC

We study the possibility of discovering and measuring the properties of the lightest Kaluza-Klein excitation of the gluon in a Randall-Sundrum scenario where the Standard Model matter and gauge fields propagate in the bulk. The KK-gluon decays primarily into top quarks. We discuss how to use the $t \bar{t}$ final states to discover and probe the properties of the KK-gluon. Identification of highly energetic tops is crucial for this analysis. We show that conventional identification methods relying on well separated decay products will not work for heavy resonances but suggest alternative methods for top identification for energetic tops. We find, conservatively, that resonances with masses less than 5 TeV can be discovered if the algorithm to identify high $p_T$ tops can reject the QCD background by a factor of 10. We also find that for similar or lighter masses the spin can be determined and for lighter masses the chirality of the coupling to $t\bar t$ can be measured. Since the energetic top pair final state is a generic signature for a large class of new physics as the top quark presumably couples most strongly to the electroweak symmetry breaking sector, the methods we have outlined to study the properties of the KK-gluon should also be important in other scenarios.Comment: 21 pages, 13 figure

An Empirical Examination of Maximum Entropy Estimation.

Maximum entropy estimation is a relatively new estimation technique in econometrics. We carry out several Monte Carlo experiments using real data as a basis in order to understand the properties of the maximum entropy estimator. We compare the maximum entropy and generalized maximum entropy estimators to traditional estimation techniques in linear regression, binary choice, and multinomial choice models. In addition, we discuss maximum entropy estimation in censored and truncated regression models. We find that the generalized maximum entropy estimator dominates the logit estimator and the multinomial logit estimator in Monte Carlo experiments. The generalized maximum entropy estimator in discrete choice models allows us to jointly estimate the unknown probabilities and the unknown errors resulting in more uniform predicted probabilities and reducing the variance of the parameter estimates. In the linear regression problem, the generalized maximum entropy estimator allows us to impose nonsample information about the unknown parameters and errors. However, we must impose a set of support points for unknown parameters and errors, which is not always an easy thing to do. We find that when we do specify nonsample information that is correct, the generalized maximum entropy estimator has lower risk than either the ordinary least squares or the inequality restricted least squares estimators. From our sampling experiments using real data, we find that maximum entropy estimation is a viable estimation technique in several econometric models

Academic Success and the Transfer of Community College Credits in the Principles of Economics

A growing number of today’s college students attend local 2-year community colleges. Many of these students will ultimately transfer to major universities in pursuit of the traditional Bachelors degree. The question of whether such transfer credits adequately prepare students for future academic endeavors is important for educators interested in preparing successful students and maintaining the quality of their institution. In this paper, we examine whether students who transfer credits earned for the traditional Principles of Economics course sequence achieve the same levels of academic success, measured in terms of GPA, as students taking the sequence at a major state university. The model indicates that community college transfer students perform poorly relative to native students in terms of cumulative GPA. This result is driven by a self-selection process whereby the more academically challenged students are those who choose to transfer credit from 2-year schools. The results of our model are used to develop a grade equivalency measure between the university and 2-year schools. Using this measure we are able to reject the hypothesis that grades are equivalent between 2- and 4-year institutions. Finally, we find that grades in the Principles of Economics sequence are strong predictors of overall academic success

Semantic properties and the computational model of mind.

Much of the contemporary research in cognitive psychology presupposes an information processing or computational model of human cognitive processes. On this view cognitive states are characterized as relations to internally inscribed representations. Jerry Fodor and Zenon Pylyshyn have argued that those representations have a combinatorial syntax and a compositional semantics, and Fodor has argued that the individuation of representations according to semantic type corresponds, roughly, to individuation according to syntactic type. I investigate whether this computational model requires us to appeal, directly or indirectly, to the semantic properties of representations when we explain cognitive behavior. I first discuss the requirements of scientific explanation in general, and the constraints of materialism and physicalism in particular. Then I outline how it is possible for semantic entities to be involved in cognitive explanations, and how Fodor and Pylyshyn think they are involved in explanations on the computational model. I consider whether, given the computational model, references to representations are necessary to explain cognitive processes or whether references to representations can be eliminated in favor or references to uninterpreted formulae. Finally I criticize the argument, suggested by both Fodor and Pylyshyn, that it is our ability to respond to nonnomic or nonprojectable properties of stimuli that requires explanation in terms of the semantic properties of representations

Preparation of pigments for space-stable thermal control coatings Interim summary report, 1 Jun. 1968 - 30 Apr. 1969

Control of vapor phase reaction kinetics to produce pigments by homogeneous nucleatio

Impulsive Spin-Motion Entanglement for Fast Quantum Computation and Sensing

We perform entanglement of spin and motional degrees of freedom of a single, ground-state trapped ion through the application of a $16$ ps laser pulse. The duration of the interaction is significantly shorter than both the motional timescale ($\approx 10$ $\mu$s) and spin precession timescale ($\approx 10$ ns), demonstrating that neither sets a fundamental speed limit on this operation for quantum information processing. Entanglement is demonstrated through the collapse and revival of spin coherence as the spin components of the wavefunction separate and recombine in phase space. We infer the fidelity of these single qubit operations to be $97(3)\%$.Comment: 5 pages, 4 figure