3,740 research outputs found
The Kato square root problem on vector bundles with generalised bounded geometry
We consider smooth, complete Riemannian manifolds which are exponentially
locally doubling. Under a uniform Ricci curvature bound and a uniform lower
bound on injectivity radius, we prove a Kato square root estimate for certain
coercive operators over the bundle of finite rank tensors. These results are
obtained as a special case of similar estimates on smooth vector bundles
satisfying a criterion which we call generalised bounded geometry. We prove
this by establishing quadratic estimates for perturbations of Dirac type
operators on such bundles under an appropriate set of assumptions.Comment: Slight technical modification of the notion of "GBG constant section"
on page 7, and a few technical modifications to Proposition 8.4, 8.6, 8.
Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation
Much evidence points towards that the photosphere in the relativistic outflow
in GRBs plays an important role in shaping the observed MeV spectrum. However,
it is unclear whether the spectrum is fully produced by the photosphere or
whether a substantial part of the spectrum is added by processes far above the
photosphere. Here we make a detailed study of the ray emission from
single pulse GRB110920A which has a spectrum that becomes extremely narrow
towards the end of the burst. We show that the emission can be interpreted as
Comptonisation of thermal photons by cold electrons in an unmagnetised outflow
at an optical depth of . The electrons receive their energy by a
local dissipation occurring close to the saturation radius. The main spectral
component of GRB110920A and its evolution is thus, in this interpretation,
fully explained by the emission from the photosphere including localised
dissipation at high optical depths.Comment: 14 pages, 11 figures, accepted to MNRA
The Right Thing at the Right Time: Why Ostensive Naming Facilitates Word Learning
The current study examines how focusing children’s attention immediately after fast mapping improves their ability to retain novel names. Previous research suggests that young children can only retain novel names presented via referent selection if ostensive naming is provided and that such explicit naming works by increasing children’s attention to the target and decreasing their attention to the competitor objects (Horst and Samuelson, 2008). This explanation of the function of ostensive naming after referent selection trials was tested by drawing 24-month-old children’s attention to the target either by illuminating the target, covering the competitors, or both. A control group was given a social pragmatic cue (pointing). Children given social pragmatic cue support did not demonstrate retention. However, children demonstrated retention if the target object was illuminated, and also when it was illuminated and the competitors simultaneously dampened. This suggests that drawing children’s attention to the target object in a manner that helps focus children’s attention is critical for word learning via referent selection. Directing attention away from competitors while also directing attention toward a target also aids in the retention of novel words
Learning what to remember: vocabulary knowledge and children’s memory for object names and features
Although young children can map a novel name to a novel object, it remains unclear what they actually remember about objects when they initially make such a name-object association. In the current study we investigated 1) what children remembered after they were initially introduced to name-object associations and 2) how their vocabulary size and vocabulary structure influenced what they remembered. As a group, children had difficulty remembering each of the features of the original novel objects. Further analyses revealed that differences in vocabulary structure predicted children’s ability to remember object features. Specifically, children who produced many names for categories organized by similarity in shape (e.g., ball, cup) had the best memory for newly-learned objects’ features—especially their shapes. In addition, the more features children remembered, the more likely they were to retain the newly-learned name-object associations. Vocabulary size, however, was not predictive of children’s feature memory or retention. Taken together, these findings demonstrate that children’s existing vocabulary structure, rather than simply vocabulary size, influences what they attend to when encountering a new object and subsequently their ability to remember new name-object associations
Testing a word is not a test of word learning
Although vocabulary acquisition requires children learn names for multiple things, many investigations of word learning mechanisms teach children the name for only one of the objects presented. This is problematic because it is unclear whether children's performance reflects recall of the correct name-object association or simply selection of the only object that was singled out by being the only object named. Children introduced to one novel name may perform at ceiling as they are not required to discriminate on the basis of the name per se, and appear to rapidly learn words following minimal exposure to a single word. We introduced children to four novel objects. For half the children, only one of the objects was named and for the other children, all four objects were named. Only children introduced to one word reliably selected the target object at test. This demonstration highlights the over-simplicity of one-word learning paradigms and the need for a shift in word learning paradigms where more than one word is taught to ensure children disambiguate objects on the basis of their names rather than their degree of salience
Probing the temporal variability of Cygnus X-1 into the soft state
Building on results from previous studies of Cygnus~X-1, we analyze Rossi
X-ray Timing Explorer (RXTE) data taken when the source was in the soft and
transitional spectral states. We look at the power spectrum in the 0.01 -- 50
Hz range, using a model consisting of a cut-off power-law and two Lorentzian
components. We are able to constrain the relation between the characteristic
frequencies of the Lorentzian components, and show that it is consistent with a
power-law relation having the same index (1.2) as previously reported for the
hard state, but shifted by a factor ~2. Furthermore, it is shown that the
change in the frequency relation seen during the transitions can be explained
by invoking a shift of one Lorentzian component to a higher harmonic, and we
explore the possible support for this interpretation in the other component
parameters. With the improved soft state results we study the evolution of the
fractional variance for each temporal component. This approach indicates that
the two Lorentzian components are connected to each other, and unrelated to the
power-law component in the power spectrum, pointing to at least two separate
emission components.Comment: 11 pages, 10 figures, 1 electronic table. Accepted for publication in
A&
Nonlinear resonance in a three-terminal carbon nanotube resonator
The RF-response of a three-terminal carbon nanotube resonator coupled to
RF-transmission lines is studied by means of perturbation theory and direct
numerical integration. We find three distinct oscillatory regimes, including
one regime capable of exhibiting very large hysteresis loops in the frequency
response. Considering a purely capacitive transduction, we derive a set of
algebraic equations which can be used to find the output power (S-parameters)
for a device connected to transmission lines with characteristic impedance
.Comment: 16 pages, 8 figure
Visibilia ex invisibilibus: seeing at the nanoscale for improved preservation of parchment
This paper describes the application of atomic force microscopy (AFM) for the imaging of collagen denaturation as
observed in parchment. Parchment is prepared from processed animal skin and collagen is the main component. Large
collections in national archives, libraries and religious institutions contain numerous documents written on parchment.
Their preservation presents an unsolved problem for conservators. The main challenge is to assess the state of collagen
and to detect what conservators refer to as the pre-gelatinised state, which can cause surface cracking resulting in a loss
of text and can increase the vulnerability of parchment to aqueous cleaning agents. Atomic force microscopy (AFM) was
first used within the Improved Damage Assessment of Parchment (IDAP) project, enabling the characterisation of the
collagen structure within parchment at the nanoscale. Damage categories were also established based on the extent
of the ordered collagen structure that was observed in the AFM images. This paper describes the work following the
IDAP project, where morphological changes in the fibres due to both artificial and natural ageing were observed and
linked to observations made by AFM. It also explores the merits and drawbacks of different approaches used for sample
preparation and the possibility of using a portable AFM for imaging directly on the surface of documents. A case study on
a manuscript from the 18th century is presented
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