7,139 research outputs found
Lazy Abstraction-Based Controller Synthesis
We present lazy abstraction-based controller synthesis (ABCS) for
continuous-time nonlinear dynamical systems against reach-avoid and safety
specifications. State-of-the-art multi-layered ABCS pre-computes multiple
finite-state abstractions of varying granularity and applies reactive synthesis
to the coarsest abstraction whenever feasible, but adaptively considers finer
abstractions when necessary. Lazy ABCS improves this technique by constructing
abstractions on demand. Our insight is that the abstract transition relation
only needs to be locally computed for a small set of frontier states at the
precision currently required by the synthesis algorithm. We show that lazy ABCS
can significantly outperform previous multi-layered ABCS algorithms: on
standard benchmarks, lazy ABCS is more than 4 times faster
Polarization state of the optical near-field
The polarization state of the optical electromagnetic field lying several
nanometers above complex dielectric structures reveals the intricate
light-matter interaction that occurs in this near-field zone. This information
can only be extracted from an analysis of the polarization state of the
detected light in the near-field. These polarization states can be calculated
by different numerical methods well-suited to near--field optics. In this
paper, we apply two different techniques (Localized Green Function Method and
Differential Theory of Gratings) to separate each polarisation component
associated with both electric and magnetic optical near-fields produced by
nanometer sized objects. The analysis is carried out in two stages: in the
first stage, we use a simple dipolar model to achieve insight into the physical
origin of the near-field polarization state. In the second stage, we calculate
accurate numerical field maps, simulating experimental near-field light
detection, to supplement the data produced by analytical models. We conclude
this study by demonstrating the role played by the near-field polarization in
the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
Discrepancies in Determinations of the Ginzburg-Landau Parameter
Long-standing discrepancies within determinations of the Ginzburg-Landau
parameter from supercritical field measurements on superconducting
microspheres are reexamined. The discrepancy in tin is shown to result from
differing methods of analyses, whereas the discrepancy in indium is a
consequence of significantly differing experimental results. The reanalyses
however confirms the lower determinations to within experimental
uncertainties.Comment: submitted to Phys. Rev.
The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo
Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori
class star separated by au. Observational evidence indicate a complex
environment in which each star has a circumstellar disk and drives a jet, and
the whole system is embedded in a large dusty envelope.
Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400
au in scattered light.
Methods. We use high contrast imaging polarimetry with VLT/NaCo at and
bands.
Results. The central binary is resolved in both bands. The polarized images
show three bright and complex structures: a common dust envelope, a sharp
extended feature previously reported in direct light, and an intriguing bright
clump located 0\farcs3 south of the binary, which appears spatially connected
to the sharp extended feature.
Conclusions.We detect orbital motion when compared to previous observations,
and report a new outburst driven by the Herbig star. Our observations reveal
the complex inner environment of Z\,CMa with unprecedented detail and contrast.Comment: Accepted for publication in A&A Letter
A Deep Proper-Motion Survey in Kapteyn Selected Areas: I. Survey Description and First Results for Stars in the Tidal Tail of Sagittarius and in the Monoceros Ring
We describe a high-precision, deep (to V ~ 19-21) absolute proper-motion
survey that samples ~50 lines of sight in the Kapteyn Selected Areas along
declination zones -15, 0 and 15 degrees. In many fields the astrometric
baseline reaches nearly a century. We demonstrate that these data provide
typical per star precisions between ~ 1 and 3 mas/yr to the above magnitude
limits, with the absolute reference frame established by numerous extragalactic
sources in each survey field. Combined with existing and ongoing photometric
and radial velocity surveys in these fields, these astrometric data will
enable, among other things, accurate, detailed dynamical modeling of satellite
interactions with our Galaxy. In this contribution we describe the astrometric
part of our survey and show preliminary results along the trailing tail of the
Sagittarius dwarf galaxy, and in the Monoceros ring region.Comment: Accepter for publication in the Astronomical Journa
Modeling interaction of relativistic and nonrelativistic winds in binary system PSR 1259-63/SS2883. I.Hydrodynamical limit
In this paper, we present a detailed hydrodynamical study of the properties
of the flow produced by the collision of a pulsar wind with the surrounding in
a binary system. This work is the first attempt to simulate interaction of the
ultrarelativistic flow (pulsar wind) with the nonrelativistic stellar wind.
Obtained results show that the wind collision could result in the formation of
an "unclosed" (at spatial scales comparable to the binary system size) pulsar
wind termination shock even when the stellar wind ram pressure exceeds
significantly the pulsar wind kinetical pressure. Moreover, the post-shock flow
propagates in a rather narrow region, with very high bulk Lorentz factor
(). This flow acceleration is related to adiabatical losses,
which are purely hydrodynamical effects. Interestingly, in this particular
case, no magnetic field is required for formation of the ultrarelativistic bulk
outflow. The obtained results provide a new interpretation for the orbital
variability of radio, X-ray and gamma-ray signals detected from binary pulsar
system PSR 1259-63/SS2883.Comment: 11 pages, 13 figures, submitted to MNRA
Young stars in the periphery of the Large Magellanic Cloud
Despite their close proximity, the complex interplay between the two
Magellanic Clouds, the Milky Way, and the resulting tidal features, is still
poorly understood. Recent studies have shown that the Large Magellanic Cloud
(LMC) has a very extended disk strikingly perturbed in its outskirts. We search
for recent star formation in the far outskirts of the LMC, out to ~30 degrees
from its center. We have collected intermediate-resolution spectra of
thirty-one young star candidates in the periphery of the LMC and measured their
radial velocity, stellar parameters, distance and age. Our measurements confirm
membership to the LMC of six targets, for which the radial velocity and
distance values match well those of the Cloud. These objects are all young
(10-50 Myr), main-sequence stars projected between 7 and 13 degrees from the
center of the parent galaxy. We compare the velocities of our stars with those
of a disk model, and find that our stars have low to moderate velocity
differences with the disk model predictions, indicating that they were formed
in situ. Our study demonstrates that recent star formation occurred in the far
periphery of the LMC, where thus far only old objects were known. The spatial
configuration of these newly-formed stars appears ring-like with a radius of 12
kpc, and a displacement of 2.6 kpc from the LMC's center. This structure, if
real, would be suggestive of a star-formation episode triggered by an
off-center collision between the Small Magellanic Cloud and the LMC's disk.Comment: Accepted for publication in MNRA
Factorization of Numbers with the temporal Talbot effect: Optical implementation by a sequence of shaped ultrashort pulses
We report on the successful operation of an analogue computer designed to
factor numbers. Our device relies solely on the interference of classical light
and brings together the field of ultrashort laser pulses with number theory.
Indeed, the frequency component of the electric field corresponding to a
sequence of appropriately shaped femtosecond pulses is determined by a Gauss
sum which allows us to find the factors of a number
Imaging the symmetry breaking of molecular orbitals in carbon nanotubes
Carbon nanotubes have attracted considerable interest for their unique
electronic properties. They are fascinating candidates for fundamental studies
of one dimensional materials as well as for future molecular electronics
applications. The molecular orbitals of nanotubes are of particular importance
as they govern the transport properties and the chemical reactivity of the
system. Here we show for the first time a complete experimental investigation
of molecular orbitals of single wall carbon nanotubes using atomically resolved
scanning tunneling spectroscopy. Local conductance measurements show
spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both
semiconducting and metallic tubes, demonstrating the symmetry breaking of
molecular orbitals in nanotubes. Whatever the tube, only two types of
complementary orbitals are alternatively observed. An analytical tight-binding
model describing the interference patterns of ? orbitals confirmed by ab initio
calculations, perfectly reproduces the experimental results
Radiographic preoperative templating of extra-offset cemented THA implants: How reliable is it and how does it affect survival?
SummaryIntroductionSecuring femoral offset should in theory improve hip stability and abductor muscles moment arms. As problems arise mainly in case of originally increased offset (>40mm), a range of extra-offset stems is available; the exact impact in terms of fixation, however, is not known.HypothesisExtra-offset stems should more reliably reestablish original femoral offsets exceeding 40mm than standard femoral components, limiting instability risk without possible adverse effect on fixation.ObjectiveTo compare the ability of five commonly available femoral stem designs to restitute offset exceeding 40mm, and to assess function and cement fixation at a minimum 6 years’ follow-up in a stem conceived to reproduce such offset.Patients and methodsA continuous series of 74 total hip replacements (THR) in hips with increased (>40mm) femoral offset was studied. All underwent preoperative X-ray templating on Imagika™ software to assess offset reproduction by five models of stem: four standard, and one Lubinus SP2™ extra-offset stem. A retrospective clinical and X-ray study was conducted with a minimum 6 years’ follow-up on the Lubinus SP2™ 117° stems used to try to reproduce offset in the 74 THRs.ResultsApart from the increased (>40mm) offset, the cervicodiaphyseal angle was consistently <135°, <130° in 60 femurs (81%) and <125° in 45 (60%). Planning showed the four standard stems to induce (>5mm femoral offset reduction in 50–83% of cases, versus only 25% with the Lubinus SP2™ 117°). All 74 hips received Lubinus SP2™ 117° stems: at a mean 78 months FU (range, 70–94mo), their mean Postel-Merle d’Aubigné score was 17±1.8 (range, 13–18). Five of the 74 THRs underwent surgical revision: three cases of loosening, in which the stem was replaced, and two of instability, without change of stem. Loosening was not related to offset reproduction quality; two of the three cases were due to initial cementing defect, and the third occurred in a femur with previous history of two osteotomies. There were four cases of dislocation (5.4%: two primary, which were not operated on, and two recurrent, managed by acetabular revision), despite good reproduction of the preoperative offset in three of the four cases. Mean 7-year implant survivorship was 95.1% (±4.8).Discussion and conclusionThe anatomic form of the Lubinus™ SP2 117° should in theory provide a uniform cement mantle. Survivorship, however, is less good than for regular offset versions (126° or 135°). On the other hand, it does reproduce anatomy in case of >40mm offset, providing extra offset of more than 51mm. The slightly shorter survivorship requires more long-term surveillance.Level of evidenceLevel IV, retrospective study
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