14,176 research outputs found
The Homflypt polynomial and the oriented Thompson group
We show how to construct unitary representations of the oriented Thompson
group from oriented link invariants. In particular we show that the
suitably normalised HOMFLYPT polynomial defines a positive definite function of
.Comment: To appear in Quantum Topolog
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Short-term memory and vocabulary development in children with Down syndrome and children with specific language impairment
A longitudinal comparison was made between development of verbal and visuo-spatial short-term memory and vocabulary in children with Down syndrome (DS), children with specific language impairment (SLI), and typically developing children as a control group. Participants were 12 children with DS (6 males, 6 females; mean chronological age 9y 9mo [SD 2.8 mo], range 8y 6mo to 11y 4mo); nine children with SLI (4 males, 5 females; mean chronological age 3y 9mo [SD 4.8mo], range 3y 3mo to 4y 5mo); and 12 typically developing children (5 males, 7 females; mean chronological age 4y 4mo [SD 3.9mo], range 3y 3mo to 4y 3mo). Participants were matched on mental age (mean mental age 4y 3mo). All participants completed verbal short-term memory, visuo-spatial short-term memory, and expressive and receptive vocabulary tasks on three occasions over 1 year. Similarities were seen in the clinical groups for verbal short-term memory. There was some evidence of difficulty in visuo-spatial short-term memory in the children with SLI relative to the other groups, but all three groups showed overlap in visuo-spatial short-term memory performance. At the final time-point vocabulary performance in the clinical groups was similar; the typically developing children showed higher vocabulary abilities than both clinical groups
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Cognitive abilities in children with specific language impairment: consideration of visuo-spatial skills
Background: The study is concerned with the cognitive abilities of children with specific language impairment (SLI). Previous research has indicated that children with SLI demonstrate difficulties with certain cognitive tasks despite normal nonâverbal IQ scores. It has been suggested that a general processing limitation might account for the pattern of language and cognitive difficulties seen in children with SLI. The performances on a visuoâspatial shortâterm memory task and a visuoâspatial processing task were considered in a group of young children with SLI. Verbal shortâterm memory was also measured.
Aims: To identify whether children with SLI demonstrate difficulties with visuoâspatial memory as well as verbal shortâterm memory. To see whether a visuoâspatial processing task without shortâterm memory requirements is problematic for children with SLI. To consider performance on these tasks over time.
Methods & Procedures: Nine children with SLI (mean age 3;9 years at the study outset) and nine typically developing children (mean age 3;9 years at the study outset) were visited on three occasions over 1 year. Verbal shortâterm memory, visuoâspatial shortâterm memory and visuoâspatial processing tasks were administered to the children, and performance over time was compared between the two groups.
Outcomes & Results: The children with SLI performed at a lower level than the typically developing children on the verbal shortâterm memory task. Both groups showed similar development on the verbal shortâterm memory task and the visuoâspatial processing task over time. Only the visuoâspatial shortâterm memory task showed slower development over time in the children with SLI relative to the typically developing children.
Conclusions: Children with SLI demonstrated slower development on a visuoâspatial shortâterm memory task relative to typically developing children of the same chronological age. This finding has implications for speech and language therapists and other professionals working with children with SLI. It may mean that only certain types of visual support are suitable, and that children with SLI will have difficulty with tasks requiring a high level of processing, or a number of mental manipulations
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Evaluation and comparison of satellite precipitation estimates with reference to a local area in the Mediterranean Sea
Precipitation is one of the major variables for many applications and disciplines related to water resources and the geophysical Earth system. Satellite retrieval systems, rain-gauge networks, and radar systems are complementary to each other in terms of their coverage and capability of monitoring precipitation. Satellite-rainfall estimate systems produce data with global coverage that can provide information in areas for which data from other sources are unavailable. Without referring to ground measurements, satellite-based estimates can be biased and, although some gauge-adjusted satellite-precipitation products have been already developed, an effective way of integrating multi-sources of precipitation information is still a challenge.In this study, a specific area, the Sicilia Island (Italy), has been selected for the evaluation of satellite-precipitation products based on rain-gauge data. This island is located in the Mediterranean Sea, with a particular climatology and morphology, which can be considered an interesting test site for satellite-precipitation products in the European mid-latitude area. Four satellite products (CMORPH, PERSIANN, PERSIANN-CCS, and TMPA-RT) and two GPCP-adjusted products (TMPA and PERSIANN Adjusted) have been selected. Evaluation and comparison of selected products is performed with reference to data provided by the rain-gauge network of the Island Sicilia and by using statistical and graphical tools. Particular attention is paid to bias issues shown both by only-satellite and adjusted products. In order to investigate the current and potential possibilities of improving estimates by means of adjustment procedures using GPCC ground precipitation, the data have been retrieved separately and compared directly with the reference rain-gauge network data set of the study area.Results show that bias is still considerable for all satellite products, then some considerations about larger area climatology, PMW-retrieval algorithms, and GPCC data are discussed to address this issue, along with the spatial and seasonal characterization of results. © 2013 Elsevier B.V
Solvable Lie algebras are not that hypo
We study a type of left-invariant structure on Lie groups, or equivalently on
Lie algebras. We introduce obstructions to the existence of a hypo structure,
namely the 5-dimensional geometry of hypersurfaces in manifolds with holonomy
SU(3). The choice of a splitting g^*=V_1 + V_2, and the vanishing of certain
associated cohomology groups, determine a first obstruction. We also construct
necessary conditions for the existence of a hypo structure with a fixed
almost-contact form. For non-unimodular Lie algebras, we derive an obstruction
to the existence of a hypo structure, with no choice involved. We apply these
methods to classify solvable Lie algebras that admit a hypo structure.Comment: 21 pages; v2: presentation improved, typos corrected, notational
conflicts eliminated. To appear in Transformation Group
Electron Correlation and Charge Transfer Instability in Bilayered Two Dimensional Electron Gas
We prove that the predicted charge transfer state in symmetric bilayers of
two dimensional electron gases is always unstable at zero bias voltage, due to
interlayer correlation and/or tunneling. This is most easily seen by resorting
to a pseudospin formalism and considering coherent states obtained from the
charge transfer state through rotations of the pseudospins. Evidently, the
charge transfer state is stabilized by a sufficiently strong gate voltage, as
found in recent experiments. We show that a simple model, in which the layers
are strictly two dimensional, is able to account quantitatively for such
experimental findings, when correlation is properly included.Comment: 5 pages, 3 figures. Subm. to Europhys. Let
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