181,195 research outputs found
Reduced classes and curve counting on surfaces II: calculations
We calculate the stable pair theory of a projective surface . For fixed
curve class the results are entirely topological, depending
on , , , , \emph{and}
invariants of the ring structure on such as the Pfaffian of
considered as an element of . Amongst other things, this
proves an extension of the G\"ottsche conjecture to non-ample linear systems.
We also give conditions under which this calculates the full 3-fold reduced
residue theory of . This is related to the reduced residue Gromov-Witten
theory of via the MNOP conjecture. When the surface has no holomorphic
2-forms this can be expressed as saying that certain Gromov-Witten invariants
of are topological.
Our method uses the results of \cite{KT1} to express the reduced virtual
cycle in terms of Euler classes of bundles over a natural smooth ambient space.Comment: 19 pages. Minor correction
The potential for bias in principal causal effect estimation when treatment received depends on a key covariate
Motivated by a potential-outcomes perspective, the idea of principal
stratification has been widely recognized for its relevance in settings
susceptible to posttreatment selection bias such as randomized clinical trials
where treatment received can differ from treatment assigned. In one such
setting, we address subtleties involved in inference for causal effects when
using a key covariate to predict membership in latent principal strata. We show
that when treatment received can differ from treatment assigned in both study
arms, incorporating a stratum-predictive covariate can make estimates of the
"complier average causal effect" (CACE) derive from observations in the two
treatment arms with different covariate distributions. Adopting a Bayesian
perspective and using Markov chain Monte Carlo for computation, we develop
posterior checks that characterize the extent to which incorporating the
pretreatment covariate endangers estimation of the CACE. We apply the method to
analyze a clinical trial comparing two treatments for jaw fractures in which
the study protocol allowed surgeons to overrule both possible randomized
treatment assignments based on their clinical judgment and the data contained a
key covariate (injury severity) predictive of treatment received.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS477 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Large wind turbine generators
The development associated with large wind turbine systems is briefly described. The scope of this activity includes the development of several large wind turbines ranging in size from 100 kW to several megawatt levels. A description of the wind turbine systems, their programmatic status and a summary of their potential costs is included
Understanding the effects of geometry and rotation on pulsar intensity profiles
We have developed a method to compute the possible distribution of radio
emission regions in a typical pulsar magnetosphere, taking into account the
viewing geometry and rotational effects of the neutron star. Our method can
estimate the emission altitude and the radius of curvature of particle
trajectory as a function of rotation phase for a given inclination angle,
impact angle, spin-period, Lorentz factor, field line constant and the
observation frequency. Further, using curvature radiation as the basic emission
mechanism, we simulate the radio intensity profiles that would be observed from
a given distribution of emission regions, for different values of radio
frequency and Lorentz factor. We show clearly that rotation effects can
introduce significant asymmetries into the observed radio profiles. We
investigate the dependency of profile features on various pulsar parameters. We
find that the radiation from a given ring of field lines can be seen over a
large range of pulse longitudes, originating at different altitudes, with
varying spectral intensity. Preferred heights of emission along discrete sets
of field lines are required to reproduce realistic pulsar profiles, and we
illustrate this for a known pulsar. Finally, we show how our model provides
feasible explanations for the origin of core emission, and also for one-sided
cones which have been observed in some pulsars.Comment: 21 pages, 11 figures, accepted for publication in MNRA
Isospectral But Physically Distinct: Modular Symmetries and their Implications for Carbon Nanotori
Recently there has been considerable interest in the properties of carbon
nanotori. Such nanotori can be parametrized according to their radii, their
chiralities, and the twists that occur upon joining opposite ends of the
nanotubes from which they are derived. In this paper, however, we demonstrate
that many physically distinct nanotori with wildly different parameters
nevertheless share identical band structures, energy spectra, and electrical
conductivities. This occurs as a result of certain geometric symmetries known
as modular symmetries which are direct consequences of the properties of the
compactified graphene sheet. Using these symmetries, we show that there is a
dramatic reduction in the number of spectrally distinct carbon nanotori
compared with the number of physically distinct carbon nanotori. The existence
of these modular symmetries also allows us to demonstrate that many statements
in the literature concerning the electronic properties of nanotori are
incomplete because they fail to respect the spectral equivalences that follow
from these symmetries. We also find that as a result of these modular
symmetries, the fraction of spectrally distinct nanotori which are metallic is
approximately three times greater than would naively be expected on the basis
of standard results in the literature. Finally, we demonstrate that these
modular symmetries also extend to cases in which our carbon nanotori enclose
non-zero magnetic fluxes.Comment: 12 pages, ReVTeX, 6 figures, 1 table. Replaced to match published
versio
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