776 research outputs found
Growth and order of automorphisms of free groups and free Burnside groups
We prove that an outer automorphism of the free group is exponentially
growing if and only if it induces an outer automorphism of infinite order of
free Burnside groups with sufficiently large odd exponent.Comment: 36 pages, 4 figure
Should we sync? Seascape-level genetic and ecological factors determine seagrass flowering patterns
3 figuras, 2 tablas1. Spatial and temporal heterogeneity in flowering occur in many plant species with abiotic
pollination and may confer fitness advantages through mechanisms such as predator
satiation or pollination efficiency. Environmental factors such as light quality or quantity and
temperature play an important role in inducing synchronisation on wide geographic scales.
On a smaller geographic scale, external factors such as resource availability and herbivory
are theorised to trigger flowering, while genetic factors may also play an important role.
2. In this study, we assessed the importance of ecological and genetic factors in shaping
seascape-level spatial heterogeneity in flowering of the seagrass Posidonia oceanica. By
investigating spatially close sites (<20 km) with similar seascape configurations and depth,
we assume that major environmental drivers (temperature and light) were equivalent.
3. We assessed four ecological factors (productivity, leaf nitrogen and carbon content and
herbivory) and three genetic factors (heterozygosity, relatedness and clonality) to assess
three hypotheses for synchronised flowering in P. oceanica: (1) clone synchronisation
(internal clock hypothesis), (2) variation in nutrient availability, potentially caused by spatial
heterogeneity in herbivory rates or nutrient translocation via clonal integration (resource
budget hypothesis) or (3) kin selection and sibling synchronisation. 4. Internal relatedness and heterozygosity had a significant positive effect on the abundance of
flowers. Moreover, productivity and genotypic richness (clonality) were negatively
associated with flower density, although at a lower level of significance. In addition we
found that clones were almost exclusively shared among mass-flowering patches and
patches without mass-flowering, respectively.
5. Synthesis. The results shed new light on seagrass flowering patterns and on the mechanisms
of flower synchronisation at the patch level within a wider spatial scale. We found support
for the kin selection hypothesis and indirect evidence for the resource budget hypothesis.
Thus a combination of mainly genetic but also ecological factors causes the observed
heterogeneous flowering patterns in Posidonia oceanica seascapes. In addition, we found a
strong positive relationship between the number of flowers and heterozygosity, adding
evidence to the controversial association between heterozygosity and fitness when a limited
number of loci are used. To our knowledge, this study is the first to link both ecological and
genetic factors with flower abundance in a species with a presumed masting strategy.This
research was partially supported by the European Community’s 7th Framework Programme
(FP7/2007–2013) CoCoNet, the MIUR Italian Flagship project RITMARE, the Spanish Ministry of
Science and Innovation (CTM2010-22273-C02-02) and the Spanish National Research Council
(PIE201330EO62). M.J. is supported by a SZN PhD fellowship via the Open University and the Spanish
Ministry of Education supported J.P. (scholarship AP2008- 01601).Peer reviewe
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of
continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a
fully coherent search, based on matched filtering, which uses the position and rotational parameters
obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto-
noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch
between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have
been developed, allowing a fully coherent search for gravitational waves from known pulsars over a
fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of
11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial
outliers, further studies show no significant evidence for the presence of a gravitational wave signal.
Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of
the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for
the first time. For an additional 3 targets, the median upper limit across the search bands is below the
spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried
out so far
Tracking Membrane Protein Association in Model Membranes
Membrane proteins are essential in the exchange processes of cells. In spite of great breakthrough in soluble proteins studies, membrane proteins structures, functions and interactions are still a challenge because of the difficulties related to their hydrophobic properties. Most of the experiments are performed with detergent-solubilized membrane proteins. However widely used micellar systems are far from the biological two-dimensions membrane. The development of new biomimetic membrane systems is fundamental to tackle this issue
Length Sensing and Control in the Virgo Gravitational Wave Interferometer
The gravitational wave detector Virgo is presently being commissioned. A significant part of last year was spent in setting up the cavity length control system. This work was carried out with steps of increasing complexity: locking a simple Fabry-Perot cavity, then a Michelson interferometer with Fabry-Perot cavities in both arms, and finally recycling the light beam into the interferometer. The applied strategy and the main results obtained are describe
Low loss coatings for the VIRGO large mirrors
présentée par L. PinardThe goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances. Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization). The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performances, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction)
The gravitational wave detector VIRGO
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