997 research outputs found
Expansive actions on uniform spaces and surjunctive maps
We present a uniform version of a result of M. Gromov on the surjunctivity of
maps commuting with expansive group actions and discuss several applications.
We prove in particular that for any group and any field \K, the
space of -marked groups such that the group algebra \K[G] is
stably finite is compact.Comment: 21 page
Growth Series and Random Walks on Some Hyperbolic Graphs
Consider the tesselation of the hyperbolic plane by m-gons, l per vertex. In
its 1-skeleton, we compute the growth series of vertices, geodesics, tuples of
geodesics with common extremities. We also introduce and enumerate "holly
trees", a family of reduced loops in these graphs. We then apply Grigorchuk's
result relating cogrowth and random walks to obtain lower estimates on the
spectral radius of the Markov operator associated with a symmetric random walk
on these graphs.Comment: 21 pages. to appear in monash. mat
Finite Gel'fand pairs and their applications to Probability and Statistics
We present a general introduction to finite Gel’fand pairs and their associated spherical functions
yielding different characterizations, examine a few explicit examples, and, for each of these examples,
analyze the corresponding probabilistic problem, which will then be solved by applying the general results
and the machinery developed for a particular Gel’fand pair
Ion dynamics and coherent structure formation following laser pulse self-channeling
The propagation of a superintense laser pulse in an underdense, inhomogeneous
plasma has been studied numerically by two-dimensional particle-in-cell
simulations on a time scale extending up to several picoseconds. The effects of
the ion dynamics following the charge-displacement self-channeling of the laser
pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of
the plasma channel walls, causing an inversion of the radial space-charge field
and the filamentation of the laser pulse. At later times a number of
long-lived, quasi-periodic field structures are observed and their dynamics is
characterized with high resolution. Inside the plasma channel, a pattern of
electric and magnetic fields resembling both soliton- and vortex-like
structures is observed.Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download
a high-resolution version), to appear in Plasma Physics and Controlled Fusion
(Dec. 2007), special issue containing invited papers from the 34th EPS
Conference on Plasma Physics (Warsaw, July 2007
Potentials and limitations of NFIs and remote sensing in the assessment of harvest rates: a reply to Breidenbach et al.
AbstractThe timely and accurate monitoring of forest resources is becoming of increasing importance in light of the multi-functionality of these ecosystems and their increasing vulnerability to climate change. Remote sensing observations of tree cover and systematic ground observations from National Forest Inventories (NFIs) represent the two major sources of information to assess forest area and use. The specificity of two methods is calling for an in-depth analysis of their strengths and weaknesses and for the design of novel methods emerging from the integration of satellite and surface data. On this specific debate, a recent paper by Breidenbach et al. published in this journal suggests that the detection of a recent increase in EU forest harvest rate—as reported in Nature by Ceccherini et al.—is largely due to technical limitations of satellite-based mapping. The article centers on the difficulty of the approaches to estimate wood harvest based on remote sensing. However, it does not discuss issues with the robustness of validation approaches solely based on NFIs. Here we discuss the use of plot data as a validation set for remote sensing products, discussing potentials and limitations of both NFIs and remote sensing, and how they can be used synergistically. Finally, we highlight the need to collect in situ data that is both relevant and compatible with remote sensing products within the European Union
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