416 research outputs found
The asymptotics of an amplitude for the 4-simplex
An expression for the oscillatory part of an asymptotic formula for the
relativistic spin network amplitude for a 4-simplex is given. The amplitude
depends on specified areas for each two-dimensional face in the 4-simplex. The
asymptotic formula has a contribution from each flat Euclidean metric on the
4-simplex which agrees with the given areas. The oscillatory part of each
contribution is determined by the Regge calculus Einstein action for that
geometry.Comment: 5 pages amstex, typos correcte
Tullio Regge's legacy: Regge calculus and discrete gravity
The review paper "Discrete Structures in Physics", written in 2000, describes
how Regge's discretization of Einstein's theory has been applied in classical
relativity and quantum gravity. Here, developments since 2000 are reviewed
briefly, with particular emphasis on progress in quantum gravity through spin
foam models and group field theories.Comment: 15 pages; a contribution to the forthcoming volume "Tullio Regge: an
eclectic genius, from quantum gravity to computer play", Eds. L Castellani,
A. Ceresole, R. D'Auria and P. Fr\`e (World Scientific); v2: added references
to more relevant work, minor changes to the tex
A note on area variables in Regge calculus
We consider the possibility of setting up a new version of Regge calculus in
four dimensions with areas of triangles as the basic variables rather than the
edge-lengths. The difficulties and restrictions of this approach are discussed.Comment: 4 pages, amstex. Revision has minor changes and more precise
conclusion
Constraints on Area Variables in Regge Calculus
We describe a general method of obtaining the constraints between area
variables in one approach to area Regge calculus, and illustrate it with a
simple example. The simplicial complex is the simplest tessellation of the
4-sphere. The number of independent constraints on the variations of the
triangle areas is shown to equal the difference between the numbers of
triangles and edges, and a general method of choosing independent constraints
is described. The constraints chosen by using our method are shown to imply the
Regge equations of motion in our example.Comment: Typographical errors correcte
The asymptotics of an amplitude for the 4-simplex
An expression for the oscillatory part of an asymptotic formula for the relativistic spin network amplitude for a 4-simplex is given. The amplitude depends on specified areas for each two-dimensional face in the 4-simplex. The asymptotic formula has a contribution from each flat Euclidean metric on the 4-simplex which agrees with the given areas. The oscillatory part of each contribution is determined by the Regge calculus Einstein action for that geometry
Coupleâs Relationship With Diabetes: Means and Meanings for Management Success
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102683/1/jmft322.pd
Area Regge Calculus and Discontinuous Metrics
Taking the triangle areas as independent variables in the theory of Regge
calculus can lead to ambiguities in the edge lengths, which can be interpreted
as discontinuities in the metric. We construct solutions to area Regge calculus
using a triangulated lattice and find that on a spacelike hypersurface no such
discontinuity can arise. On a null hypersurface however, we can have such a
situation and the resulting metric can be interpreted as a so-called refractive
wave.Comment: 18 pages, 1 figur
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Fabrication of free standing collagen membranes by pulsed-electrophoretic deposition.
This work reports an important new development in the production of collagen membranes, based on pulsed electrophoretic deposition (P-EPD), suitable for a wide range of biomedical applications. Collagen membranes are of great interest as a biomaterial and in a range of other industries, though current production techniques suffer from limitations with scaling up, homogeneity, and complex shapes. P-EPD can be used to rapidly create detachable, large-area, homogeneous products with controlled thickness in a wide variety of shapes. We provide a new understanding of the influence of a range of parameters (pulse width, voltage, duty cycle, solvent additions) and their effects on membrane structure. Characterisation by AFM, SEM, and cryoSEM revealed the ability to produce dense, structurally defect-free membranes, and significantly, we show and discuss the ability to produce thicker membranes by sequential deposition without seeing a corresponding increase in cell electrical resistance. We anticipate this novel, rapid, and controllable method for the production of collagen membranes to be of interest for a wide range of fields.The authors wish to acknowledge the support of theEngineering and Physical Sciences Research Council(EPSRC)grants EP/K503009/1, EP/J500380/1, EP/L504920/1, EP/M506485/1, and EP/M508007/1,Geistlich Pharma AG, and the European ResearchCouncil(ERC)Advanced Grant 320598 3D-
Discrete structures in gravity
Discrete approaches to gravity, both classical and quantum, are reviewed
briefly, with emphasis on the method using piecewise-linear spaces. Models of
3-dimensional quantum gravity involving 6j-symbols are then described, and
progress in generalising these models to four dimensions is discussed, as is
the relationship of these models in both three and four dimensions to
topological theories. Finally, the repercussions of the generalisations are
explored for the original formulation of discrete gravity using edge-length
variables.Comment: 30 pages, 4 figure
Landmark Models for Optimizing the Use of Repeated Measurements of Risk Factors in Electronic Health Records to Predict Future Disease Risk.
The benefits of using electronic health records (EHRs) for disease risk screening and personalized health-care decisions are being increasingly recognized. Here we present a computationally feasible statistical approach with which to address the methodological challenges involved in utilizing historical repeat measures of multiple risk factors recorded in EHRs to systematically identify patients at high risk of future disease. The approach is principally based on a 2-stage dynamic landmark model. The first stage estimates current risk factor values from all available historical repeat risk factor measurements via landmark-age-specific multivariate linear mixed-effects models with correlated random intercepts, which account for sporadically recorded repeat measures, unobserved data, and measurement errors. The second stage predicts future disease risk from a sex-stratified Cox proportional hazards model, with estimated current risk factor values from the first stage. We exemplify these methods by developing and validating a dynamic 10-year cardiovascular disease risk prediction model using primary-care EHRs for age, diabetes status, hypertension treatment, smoking status, systolic blood pressure, total cholesterol, and high-density lipoprotein cholesterol in 41,373 persons from 10 primary-care practices in England and Wales contributing to The Health Improvement Network (1997-2016). Using cross-validation, the model was well-calibrated (Brier score = 0.041, 95% confidence interval: 0.039, 0.042) and had good discrimination (C-index = 0.768, 95% confidence interval: 0.759, 0.777)
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