19,391 research outputs found
The lichen flora of the Chagos Archipelago : including a comparison with other island and coastal tropical floras
The 1996 Chagos Expedition provided the first opportunity to study the archipelago’s lichen flora. Seventeen of the 55 islands were ecologically investigated, some in more detail than others, and lists and representative collections of lichens have been assembled for many of them. In all, 67 taxa have been recorded, 52 to specific level. Although the islands have a low biodiversity for cryptogamic plants, as would be expected in terms of their relatively young age, remoteness and small terrestrial surface areas, those taxa that are present are often found in abundance and play significant ecological roles. There is a good correlation between total lichen biodiversity and island size, despite the fact that Cocos nucifera is such an important substratum for cryptogamic plants and its presence on all islands studied provides a consistently high associated species count. Comparisons of lichen floras for ten island and coastal tropical areas show good correlations (based on the Sörensen Coefficient) within the Indian Ocean as would be expected, but poorer correlations exist within and between Pacific Ocean and neotropical floras. Ranked correlations between Chagos and other floras are in the sequence Maldives > Laing Island > Aldabra > Tuamotu > Pitcairn > N.Mariana & Belize > Guadeloupe > Cook. When coefficients are calculated using only the Physciaceae, different correlations and sequences are derived, but the affinities of the Indian Ocean islands remain strong. However, although the lichen flora of Chagos is characteristic for an Indian Ocean, it is dominated by pantropical species
Thermodynamics of a bouncer model: a simplified one-dimensional gas
Some dynamical properties of non interacting particles in a bouncer model are
described. They move under gravity experiencing collisions with a moving
platform. The evolution to steady state is described in two cases for
dissipative dynamics with inelastic collisions: (i) for large initial energy;
(ii) for low initial energy. For (i) we prove an exponential decay while for
(ii) a power law marked by a changeover to the steady state is observed. A
relation for collisions and time is obtained and allows us to write relevant
observables as temperature and entropy as function of either number of
collisions and time.Comment: 36 pages, 10 figures. To appear in: Communications in Nonlinear
Science and Numerical Simulation, 201
Bounds on the maximum multiplicity of some common geometric graphs
We obtain new lower and upper bounds for the maximum multiplicity of some
weighted and, respectively, non-weighted common geometric graphs drawn on n
points in the plane in general position (with no three points collinear):
perfect matchings, spanning trees, spanning cycles (tours), and triangulations.
(i) We present a new lower bound construction for the maximum number of
triangulations a set of n points in general position can have. In particular,
we show that a generalized double chain formed by two almost convex chains
admits {\Omega}(8.65^n) different triangulations. This improves the bound
{\Omega}(8.48^n) achieved by the double zig-zag chain configuration studied by
Aichholzer et al.
(ii) We present a new lower bound of {\Omega}(12.00^n) for the number of
non-crossing spanning trees of the double chain composed of two convex chains.
The previous bound, {\Omega}(10.42^n), stood unchanged for more than 10 years.
(iii) Using a recent upper bound of 30^n for the number of triangulations,
due to Sharir and Sheffer, we show that n points in the plane in general
position admit at most O(68.62^n) non-crossing spanning cycles.
(iv) We derive lower bounds for the number of maximum and minimum weighted
geometric graphs (matchings, spanning trees, and tours). We show that the
number of shortest non-crossing tours can be exponential in n. Likewise, we
show that both the number of longest non-crossing tours and the number of
longest non-crossing perfect matchings can be exponential in n. Moreover, we
show that there are sets of n points in convex position with an exponential
number of longest non-crossing spanning trees. For points in convex position we
obtain tight bounds for the number of longest and shortest tours. We give a
combinatorial characterization of the longest tours, which leads to an O(nlog
n) time algorithm for computing them
Counting Carambolas
We give upper and lower bounds on the maximum and minimum number of geometric
configurations of various kinds present (as subgraphs) in a triangulation of
points in the plane. Configurations of interest include \emph{convex
polygons}, \emph{star-shaped polygons} and \emph{monotone paths}. We also
consider related problems for \emph{directed} planar straight-line graphs.Comment: update reflects journal version, to appear in Graphs and
Combinatorics; 18 pages, 13 figure
New Internal Stress Driven on-Chip Micromachines for Extracting Mechanical Properties of Thin Films
A new concept of micromachines has been developed for measuring the
mechanical properties of thin metallic films. The actuator is a beam undergoing
large internal stresses built up during the deposition process. Al thin films
are deposited partly on the actuator beam and on the substrate. By etching the
structure, the actuator contracts and pulls the Al film. Full stress strain
curves can be generated by designing a set of micromachines with various
actuator lengths. In the present study, the displacements have been measured by
scanning electronic microscopy. The stress is derived from simple continuum
mechanics relationships. The tensile properties of Al films of various
thicknesses have been tested. A marked increase of the strength with decreasing
film thickness is observed.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
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