6,712 research outputs found
Twisted K-theory of differentiable stacks
In this paper, we develop twisted -theory for stacks, where the twisted
class is given by an -gerbe over the stack. General properties, including
the Mayer-Vietoris property, Bott periodicity, and the product structure
are derived. Our
approach provides a uniform framework for studying various twisted -theories
including the usual twisted -theory of topological spaces, twisted
equivariant -theory, and the twisted -theory of orbifolds. We also
present a Fredholm picture, and discuss the conditions under which twisted
-groups can be expressed by so-called "twisted vector bundles".
Our approach is to work on presentations of stacks, namely \emph{groupoids},
and relies heavily on the machinery of -theory (-theory) of
-algebras.Comment: 74 page
Structural selective charge transfer in iodine-doped carbon nanotubes
We have investigated iodine intercalated carbon nanostructures by extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. We discuss here the charge transfer and the iodineâcarbon interaction as a function of the carbon nanostructures (graphite, multi-walled, double-walled and single walled nanotubes). The results show that iodine is weakly adsorbed on the surface of all multi-walled nanotubes. By contrast, a charge transfer between iodine and single walled nanotubes is evidenced
PISCO2: the new speckle camera of the Nice 76-cm refractor
We present the new speckle camera PISCO2 made in 2010-2012, for the 76-cm
refractor of C\^ote d'Azur Observatory. It is a focal instrument dedicated to
the observation of visual binary stars using high angular resolution speckle
interferometry techniques to partly overcome the degradation caused by the
atmospheric turbulence. Fitted with an EMCCD detector, PISCO2 allows the
acquisition of short exposure images that are processed in real time by our
specially designed software. Two Risley prisms are used for correcting the
atmospheric dispersion. All optical settings are remotely controlled. We have
already been able to observe faint, close binary stars with angular separations
as small as 0".16, and visual magnitudes of about 16. We also have measured
some particularly difficult systems with a magnitude difference between the two
components of about 4 magnitudes. This level of performance is very promising
for the detection and study of large sets of yet unknown (or partly measured)
binaries with close separation and/or large magnitude difference.Comment: 13 pages, 12 figure
Percolation on random networks with arbitrary k-core structure
The k-core decomposition of a network has thus far mainly served as a
powerful tool for the empirical study of complex networks. We now propose its
explicit integration in a theoretical model. We introduce a Hard-core Random
Network model that generates maximally random networks with arbitrary degree
distribution and arbitrary k-core structure. We then solve exactly the bond
percolation problem on the HRN model and produce fast and precise analytical
estimates for the corresponding real networks. Extensive comparison with
selected databases reveals that our approach performs better than existing
models, while requiring less input information.Comment: 9 pages, 5 figure
Growing networks of overlapping communities with internal structure
We introduce an intuitive model that describes both the emergence of
community structure and the evolution of the internal structure of communities
in growing social networks. The model comprises two complementary mechanisms:
One mechanism accounts for the evolution of the internal link structure of a
single community, and the second mechanism coordinates the growth of multiple
overlapping communities. The first mechanism is based on the assumption that
each node establishes links with its neighbors and introduces new nodes to the
community at different rates. We demonstrate that this simple mechanism gives
rise to an effective maximal degree within communities. This observation is
related to the anthropological theory known as Dunbar's number, i.e., the
empirical observation of a maximal number of ties which an average individual
can sustain within its social groups. The second mechanism is based on a
recently proposed generalization of preferential attachment to community
structure, appropriately called structural preferential attachment (SPA). The
combination of these two mechanisms into a single model (SPA+) allows us to
reproduce a number of the global statistics of real networks: The distribution
of community sizes, of node memberships and of degrees. The SPA+ model also
predicts (a) three qualitative regimes for the degree distribution within
overlapping communities and (b) strong correlations between the number of
communities to which a node belongs and its number of connections within each
community. We present empirical evidence that support our findings in real
complex networks.Comment: 14 pages, 8 figures, 2 table
Is durum wheat-winter pea intercropping efficient to reduce pests and diseases ?
Intercropping (IC) is known as an agricultural practice which can improve the use of environmental resources (light, nutrients and water) resulting in yield advantages compared to sole cropping (SC) (Willey, 1979) particularly in low input systems. But, diseases ands pests can strongly affect both yield and grain quality in such systems. Now, numerous studies have shown significant reductions in harmful insects and on diseases in IC compared to SC of the same species (Vandermeer, 1989; Kinane and Lyngkjaer, 2002) even if others studies did not confirmed these foundings. The aim of our study was to evaluate the assumption that IC can reduce pea pests (green aphids and weevils), pea ascochyta and main durum wheat diseases (mildew, brown rust, fusarium and septoria)
Complex networks as an emerging property of hierarchical preferential attachment
Real complex systems are not rigidly structured; no clear rules or blueprints
exist for their construction. Yet, amidst their apparent randomness, complex
structural properties universally emerge. We propose that an important class of
complex systems can be modeled as an organization of many embedded levels
(potentially infinite in number), all of them following the same universal
growth principle known as preferential attachment. We give examples of such
hierarchy in real systems, for instance in the pyramid of production entities
of the film industry. More importantly, we show how real complex networks can
be interpreted as a projection of our model, from which their scale
independence, their clustering, their hierarchy, their fractality and their
navigability naturally emerge. Our results suggest that complex networks,
viewed as growing systems, can be quite simple, and that the apparent
complexity of their structure is largely a reflection of their unobserved
hierarchical nature.Comment: 12 pages, 7 figure
- âŠ