5,642 research outputs found
On convexity of solutions of ordinary differential equations
We prove a result on the convex dependence of solutions of ordinary
differential equations on an ordered finite-dimensional real vector space with
respect to the initial data.Comment: 10 page
The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments
The raw coronagraphic performance of current high-contrast imaging
instruments is limited by the presence of a quasi-static speckle (QSS)
background, resulting from instrumental non-common path errors (NCPEs). Rapid
development of efficient speckle subtraction techniques in data reduction has
enabled final contrasts of up to 10-6 to be obtained, however it remains
preferable to eliminate the underlying NCPEs at the source. In this work we
introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront
sensor suitable for real-time NCPE correction. This pupil-plane optic combines
the apodizing phase plate coronagraph with a holographic modal wavefront
sensor, to provide simultaneous coronagraphic imaging and focal-plane wavefront
sensing using the science point spread function. We first characterise the
baseline performance of the cMWS via idealised closed-loop simulations, showing
that the sensor successfully recovers diffraction-limited coronagraph
performance over an effective dynamic range of +/-2.5 radians root-mean-square
(RMS) wavefront error within 2-10 iterations. We then present the results of
initial on-sky testing at the William Herschel Telescope, and demonstrate that
the sensor is able to retrieve injected wavefront aberrations to an accuracy of
10nm RMS under realistic seeing conditions. We also find that the cMWS is
capable of real-time broadband measurement of atmospheric wavefront variance at
a cadence of 50Hz across an uncorrected telescope sub-aperture. When combined
with a suitable closed-loop adaptive optics system, the cMWS holds the
potential to deliver an improvement in raw contrast of up to two orders of
magnitude over the uncorrected QSS floor. Such a sensor would be eminently
suitable for the direct imaging and spectroscopy of exoplanets with both
existing and future instruments, including EPICS and METIS for the E-ELT.Comment: 14 pages, 12 figures: accepted for publication in Astronomy &
Astrophysic
Rapamycin promotes arterial thrombosis in vivo: implications for everolimus and zotarolimus eluting stents
Aims Drug-eluting stents (DES) may be associated with an increased risk for stent thrombosis when compared with bare-metal stents. In endothelial cells, rapamycin induces tissue factor (TF) by inhibiting the mammalian target of rapamycin (mTOR). However, the effect of mTOR inhibition on TF activity and thrombus formation in vivo has not yet been studied. Moreover, it is unclear whether second-generation DES substances everolimus and zotarolimus have an effect on endothelial TF expression. Methods and results In a mouse carotid artery photochemical injury model, rapamycin (182 ± 27.5 ”g/L) decreased time to thrombotic occlusion by 40%, increased TF activity, and abrogated p70S6K phosphorylation when compared with controls. In vitro, rapamycin, everolimus, and zotarolimus (each 10â7 mol/l) enhanced TNF-α-induced TF expression by 2.2-, 1.7-, and 2.4-fold, respectively, which was paralleled by an increase in TF surface activity. Similar to rapamycin, everolimus and zotarolimus abrogated TNF-α-induced p70S6K phosphorylation under these conditions. Conclusion Rapamycin increases TF activity and promotes arterial thrombosis in vivo at concentrations relevant in patients undergoing DES implantation; this effect may increase the thrombogenicity of DES. Since everolimus and zotarolimus augment endothelial TF expression and activity in vitro in a similar manner as rapamycin, these findings may also be relevant for second generation DE
Can we avoid high coupling?
It is considered good software design practice to organize source code into modules and to favour within-module connections (cohesion) over between-module connections (coupling), leading to the oft-repeated maxim "low coupling/high cohesion". Prior research into network theory and its application to software systems has found evidence that many important properties in real software systems exhibit approximately scale-free structure, including coupling; researchers have claimed that such scale-free structures are ubiquitous. This implies that high coupling must be unavoidable, statistically speaking, apparently contradicting standard ideas about software structure. We present a model that leads to the simple predictions that approximately scale-free structures ought to arise both for between-module connectivity and overall connectivity, and not as the result of poor design or optimization shortcuts. These predictions are borne out by our large-scale empirical study. Hence we conclude that high coupling is not avoidable--and that this is in fact quite reasonable
Multi-Parameter Entanglement in Femtosecond Parametric Down-Conversion
A theory of spontaneous parametric down-conversion, which gives rise to a
quantum state that is entangled in multiple parameters, such as
three-dimensional wavevector and polarization, allows us to understand the
unusual characteristics of fourth-order quantum interference in many
experiments, including ultrafast type-II parametric down-conversion, the
specific example illustrated in this paper. The comprehensive approach provided
here permits the engineering of quantum states suitable for quantum information
schemes and new quantum technologies.Comment: to appear in Physical Review
One-Way Entangled-Photon Autocompensating Quantum Cryptography
A new quantum cryptography implementation is presented that combines one-way
operation with an autocompensating feature that has hitherto only been
available in implementations that require the signal to make a round trip
between the users. Using the concept of advanced waves, it is shown that this
new implementation is related to the round-trip implementations in the same way
that Ekert's two-particle scheme is related to the original one-particle scheme
of Bennett and Brassard. The practical advantages and disadvantages of the
proposed implementation are discussed in the context of existing schemes.Comment: 5 pages, 1 figure; Minor edits--conclusions unchanged; accepted for
publication in Physical Review
Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution
Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication
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