452 research outputs found
On Probabilistic Applicative Bisimulation and Call-by-Value -Calculi (Long Version)
Probabilistic applicative bisimulation is a recently introduced coinductive
methodology for program equivalence in a probabilistic, higher-order, setting.
In this paper, the technique is applied to a typed, call-by-value,
lambda-calculus. Surprisingly, the obtained relation coincides with context
equivalence, contrary to what happens when call-by-name evaluation is
considered. Even more surprisingly, full-abstraction only holds in a symmetric
setting.Comment: 30 page
Four dimensional "old minimal" N=2 supersymmetrization of R^4
We write in superspace the lagrangian containing the fourth power of the Weyl
tensor in the "old minimal" d=4, N=2 supergravity, without local SO(2)
symmetry. Using gauge completion, we analyze the lagrangian in components. We
find out that the auxiliary fields which belong to the Weyl and compensating
vector multiplets have derivative terms and therefore cannot be eliminated
on-shell. Only the auxiliary fields which belong to the compensating nonlinear
multiplet do not get derivatives and could still be eliminated; we check that
this is possible in the leading terms of the lagrangian. We compare this result
to the similar one of "old minimal" N=1 supergravity and we comment on possible
generalizations to other versions of N=1,2 supergravity.Comment: 31 pages, no figures. Minor corrections. Details of the full
calculation included as an appendix. Reference adde
Constructing Self-Dual Strings
We present an ADHMN-like construction which generates self-dual string
solutions to the effective M5-brane worldvolume theory from solutions to the
Basu-Harvey equation. Our construction finds a natural interpretation in terms
of gerbes, which we develop in some detail. We also comment on a possible
extension to stacks of multiple M5-branes.Comment: 1+19 pages, presentation improved, minor corrections, published
versio
Contamination in trials of educational interventions
Objectives: To consider the effects of contamination on the magnitude and statistical significance (or precision) of the estimated effect of an educational intervention, to investigate the mechanisms of contamination, and to consider how contamination can be avoided. Data sources: Major electronic databases were searched up to May 2005. Methods: An exploratory literature search was conducted. The results of trials included in previous relevant systematic reviews were then analysed to see whether studies that avoided contamination resulted in larger effect estimates than those that did not. Experts’ opinions were elicited about factors more or less likely to lead to contamination. We simulated contamination processes to compare contamination biases between cluster and individually randomised trials. Statistical adjustment was made for contamination using Complier Average Causal Effect analytic methods, using published and simulated data. The bias and power of cluster and individually randomised trials were compared, as were Complier Average Causal Effect, intention-to-treat and per protocol methods of analysis. Results: Few relevant studies quantified contamination. Experts largely agreed on where contamination was more or less likely. Simulation of contamination processes showed that, with various combinations of timing, intensity and baseline dependence of contamination, cluster randomised trials might produce biases greater than or similar to those of individually randomised trials. Complier Average Causal Effect analyses produced results that were less biased than intention-to-treat or per protocol analyses. They also showed that individually randomised trials would in most situations be more powerful than cluster randomised trials despite contamination. Conclusions: The probability, nature and process of contamination should be considered when designing and analysing controlled trials of educational interventions in health. Cluster randomisation may or may not be appropriate and should not be uncritically assumed always to be a solution. Complier Average Causal Effect models are an appropriate way to adjust for contamination if it can be measured. When conducting such trials in future, it is a priority to report the extent, nature and effects of contamination.We are grateful to the National Health Service Research and Development National Coordinating Centre for Research Methodology for funding this research
Conformal fields in the pp-wave limit
The pp-wave (Penrose limit) in conformal field theory can be viewed as a
special contraction of the unitary representations of the conformal group. We
study the kinematics of conformal fields in this limit in a geometric approach
where the effect of the contraction can be visualized as an expansion of
space-time. We discuss the two common models of space-time as carrier spaces
for conformal fields: One is the usual Minkowski space and the other is the
coset of the conformal group over its maximal compact subgroup. We show that
only the latter manifold and the corresponding conformal representation theory
admit a non-singular contraction limit. We also address the issue of
correlation functions of conformal fields in the pp-wave limit. We show that
they have a well-defined contraction limit if their space-time dependence
merges with the dependence on the coordinates of the R symmetry group. This is
a manifestation of the fact that in the limit the space-time and R symmetries
become indistinguishable. Our results might find applications in actual
calculations of correlation functions of composite operators in N=4 super
Yang-Mills theory.Comment: LaTex, 32 pages, 1 figure, discussion of correlation functions
extended; some corrections made; references adde
Strings between branes
D-brane configurations containing fundamental strings are constructed as
classical solutions of Yang-Mills theory. The fundamental strings in these
systems stretch between D-branes. In the case of D1-branes, this construction
gives smooth (classical) resolutions of string junctions and string networks.
Using a non-abelian Yang-Mills analysis of the string current, the string
charge density is computed and is shown to have support in the region between
the D-brane world-volumes. The 't Hooft-Polyakov monopole is analyzed using
similar methods, and is shown to contain D-strings whose flux has support off
the D-brane world-volume defined by the Higgs scalar field, when this field is
interpreted in terms of a transverse dimension. The constructions presented
here are used to give a qualitative picture of tachyon condensation in the
Yang-Mills limit, where fundamental strings and lower-dimensional D-branes
arise in a volume of space-time where brane-antibrane annihilation has
occurred.Comment: 35 pages, 16 eps figures, JHEP style; v2: a comment adde
On the Behavior of the Effective QCD Coupling alpha_tau(s) at Low Scales
The hadronic decays of the tau lepton can be used to determine the effective
charge alpha_tau(m^2_tau') for a hypothetical tau-lepton with mass in the range
0 < m_tau' < m_tau. This definition provides a fundamental definition of the
QCD coupling at low mass scales. We study the behavior of alpha_tau at low mass
scales directly from first principles and without any renormalization-scheme
dependence by looking at the experimental data from the OPAL Collaboration. The
results are consistent with the freezing of the physical coupling at mass
scales s = m^2_tau' of order 1 GeV^2 with a magnitude alpha_tau ~ 0.9 +/- 0.1.Comment: 15 pages, 4 figures, submitted to Physical Review D, added
references, some text added, no results nor figures change
Introduction to M Theory and AdS/CFT Duality
An introductory survey of some of the developments that have taken place in
superstring theory in the past few years is presented. The main focus is on
three particular dualities. The first one is the appearance of an 11th
dimension in the strong coupling limit of the type IIA theory, which give rise
to M theory. The second one is the duality between the type IIB theory
compactified on a circle and M theory on a two-torus. The final topic is an
introduction to the recently proposed duality between superstring theory or M
theory on certain anti de Sitter space backgrounds and conformally invariant
quantum field theories.Comment: 26 pages; To be published in the Proceedings of a conference held in
Corfu, Greece in September 1998. v2: reference adde
Cohort studies of fat intake and the risk of breast cancer--a pooled analysis.
Cohort studies of fat intake and the risk of breast cancer--a pooled analysis. Hunter DJ, Spiegelman D, Adami HO, Beeson L, van den Brandt PA, Folsom AR, Fraser GE, Goldbohm RA, Graham S, Howe GR, et al. Department of Epidemiology, Harvard School of Public Health, Boston, MA, USA. BACKGROUND. Experiments in animals, international correlation comparisons, and case-control studies support an association between dietary fat intake and the incidence of breast cancer. Most cohort studies do not corroborate the association, but they have been criticized for involving small numbers of cases, homogeneous fat intake, and measurement errors in estimates of fat intake. METHODS. We identified seven prospective studies in four countries that met specific criteria and analyzed the primary data in a standardized manner. Pooled estimates of the relation of fat intake to the risk of breast cancer were calculated, and data from study-specific validation studies were used to adjust the results for measurement error. RESULTS. Information about 4980 cases from studies including 337,819 women was available. When women in the highest quintile of energy-adjusted total fat intake were compared with women in the lowest quintile, the multivariate pooled relative risk of breast cancer was 1.05 (95 percent confidence interval, 0.94 to 1.16). Relative risks for saturated, monounsaturated, and polyunsaturated fat and for cholesterol, considered individually, were also close to unity. There was little overall association between the percentage of energy intake from fat and the risk of breast cancer, even among women whose energy intake from fat was less than 20 percent. Correcting for error in the measurement of nutrient intake did not materially alter these findings. CONCLUSIONS. We found no evidence of a positive association between total dietary fat intake and the risk of breast cancer. There was no reduction in risk even among women whose energy intake from fat was less than 20 percent of total energy intake. In the context of the Western lifestyle, lowering the total intake of fat in midlife is unlikely to reduce the risk of breast cancer substantially
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