544 research outputs found
Effect of the clearfelling on the water quality: Example of a spruce forest on a small catchment in France
This paper presents the variation of the hydrology and the water quality of a spruce catchment, located at Mont-Lozére (France), in a mediterranean mountain climate area , in relation to the forest status during 12 years (1981-1993). Four situations were successively examined : healthy forest (1981-84), declining stand with pest (1984-87), gradual clearfelling (1987-89) and reforestation (1989-93). An undisturbed beech catchment was used to provide reference values. In the hydrological budgets, the P-Q value (as ETR) was slightly higher in the spruce catchment than in the beech one during the first period and decreased progressively in the following ones as a consequence of: (1) the declining stand of the forest and (2) the clearfelling. No change was observed for cations, and NO3 concentrations remained were very low during the whole period in the streamwater of the beech catchment , in relation to the steady state of that ecosystem. Iin the spruce catchment,the concentrations of cations and NO, were always higher, and increased slightly during the disease. During the clearfelling, NO3 was strongly related to Ca and Mg. Six months after the reforestation, NO,, Ca , Mg concentrations were respectively 11,9 , 2,6 and 3,6 higher than at the begiming of the clearfelling.They retumed to previous values at the end of 1993. The Input-Output budget of cations presented a continuous storage in the beech catchment and simultanously a permanent release in the spruce catchment . The mean loss, -expressed as the denudation cation rate, in keq.ha-1.year-1 was as follow: -0,41 (1981-84), -0,65 (1984-87), -1,60 (1987-89) and -0,82 (1989-93).The leaching was observed during more than 6 years after the clearfelling, resulting probably from the duration of the drought period , and from the mineralization of the remaining important organic matter comparhnen
Pluricomplex Green and Lempert functions for equally weighted poles
For a domain in , the pluricomplex Green function with
poles is defined as .
When there is only one pole, or two poles in the unit ball, it turns out to be
equal to the Lempert function defined from analytic disks into by . It is known
that we always have . In the more general case where we
allow weighted poles, there is a counterexample to equality due to Carlehed and
Wiegerinck, with equal to the bidisk.
Here we exhibit a counterexample using only four distinct equally weighted
poles in the bidisk. In order to do so, we first define a more general notion
of Lempert function "with multiplicities", analogous to the generalized Green
functions of Lelong and Rashkovskii, then we show how in some examples this can
be realized as a limit of regular Lempert functions when the poles tend to each
other. Finally, from an example where in the case of
multiple poles, we deduce that distinct (but close enough) equally weighted
poles will provide an example of the same inequality. Open questions are
pointed out about the limits of Green and Lempert functions when poles tend to
each other.Comment: 25 page
Approximation of conformal mappings using conformally equivalent triangular lattices
Consider discrete conformal maps defined on the basis of two conformally
equivalent triangle meshes, that is edge lengths are related by scale factors
associated to the vertices. Given a smooth conformal map , we show that it
can be approximated by such discrete conformal maps . In
particular, let be an infinite regular triangulation of the plane with
congruent triangles and only acute angles (i.e.\ ). We scale this
tiling by and approximate a compact subset of the domain of
with a portion of it. For small enough we prove that there exists a
conformally equivalent triangle mesh whose scale factors are given by
on the boundary. Furthermore we show that the corresponding discrete
conformal maps converge to uniformly in with error of
order .Comment: 14 pages, 3 figures; v2 typos corrected, revised introduction, some
proofs extende
Neoadjuvant chemoradiation and pancreaticoduodenectomy for initially locally advanced head pancreatic adenocarcinoma
International audienceThe most accepted treatment for locally advanced pancreatic adenocarcinoma (LAPA) is chemoradiotherapy (CRT). We sought to determine the benefit of pancreaticoduodenectomy (PD) in patients with LAPA initially treated by neoadjuvant CRT
Forced Stratified Turbulence: Successive Transitions with Reynolds Number
Numerical simulations are made for forced turbulence at a sequence of
increasing values of Reynolds number, R, keeping fixed a strongly stable,
volume-mean density stratification. At smaller values of R, the turbulent
velocity is mainly horizontal, and the momentum balance is approximately
cyclostrophic and hydrostatic. This is a regime dominated by so-called pancake
vortices, with only a weak excitation of internal gravity waves and large
values of the local Richardson number, Ri, everywhere. At higher values of R
there are successive transitions to (a) overturning motions with local
reversals in the density stratification and small or negative values of Ri; (b)
growth of a horizontally uniform vertical shear flow component; and (c) growth
of a large-scale vertical flow component. Throughout these transitions, pancake
vortices continue to dominate the large-scale part of the turbulence, and the
gravity wave component remains weak except at small scales.Comment: 8 pages, 5 figures (submitted to Phys. Rev. E
Discrete complex analysis on planar quad-graphs
We develop a linear theory of discrete complex analysis on general
quad-graphs, continuing and extending previous work of Duffin, Mercat, Kenyon,
Chelkak and Smirnov on discrete complex analysis on rhombic quad-graphs. Our
approach based on the medial graph yields more instructive proofs of discrete
analogs of several classical theorems and even new results. We provide discrete
counterparts of fundamental concepts in complex analysis such as holomorphic
functions, derivatives, the Laplacian, and exterior calculus. Also, we discuss
discrete versions of important basic theorems such as Green's identities and
Cauchy's integral formulae. For the first time, we discretize Green's first
identity and Cauchy's integral formula for the derivative of a holomorphic
function. In this paper, we focus on planar quad-graphs, but we would like to
mention that many notions and theorems can be adapted to discrete Riemann
surfaces in a straightforward way.
In the case of planar parallelogram-graphs with bounded interior angles and
bounded ratio of side lengths, we construct a discrete Green's function and
discrete Cauchy's kernels with asymptotics comparable to the smooth case.
Further restricting to the integer lattice of a two-dimensional skew coordinate
system yields appropriate discrete Cauchy's integral formulae for higher order
derivatives.Comment: 49 pages, 8 figure
Single and vertically coupled type II quantum dots in a perpendicular magnetic field: exciton groundstate properties
The properties of an exciton in a type II quantum dot are studied under the
influence of a perpendicular applied magnetic field. The dot is modelled by a
quantum disk with radius , thickness and the electron is confined in the
disk, whereas the hole is located in the barrier. The exciton energy and
wavefunctions are calculated using a Hartree-Fock mesh method. We distinguish
two different regimes, namely (the hole is located at the radial
boundary of the disk) and (the hole is located above and below the
disk), for which angular momentum transitions are predicted with
increasing magnetic field. We also considered a system of two vertically
coupled dots where now an extra parameter is introduced, namely the interdot
distance . For each and for a sufficient large magnetic field,
the ground state becomes spontaneous symmetry broken in which the electron and
the hole move towards one of the dots. This transition is induced by the
Coulomb interaction and leads to a magnetic field induced dipole moment. No
such symmetry broken ground states are found for a single dot (and for three
vertically coupled symmetric quantum disks). For a system of two vertically
coupled truncated cones, which is asymmetric from the start, we still find
angular momentum transitions. For a symmetric system of three vertically
coupled quantum disks, the system resembles for small the pillar-like
regime of a single dot, where the hole tends to stay at the radial boundary,
which induces angular momentum transitions with increasing magnetic field. For
larger the hole can sit between the disks and the state
remains the groundstate for the whole -region.Comment: 11 pages, 16 figure
Expansion du pin d'Alep. RÎle des processus allélopathiques dans la dynamique successionnelle.
L'objet de cet article est de montrer quel peut ĂȘtre le rĂŽle des processus allĂ©lopathiques dans le cas d'espĂšces ligneuses constituant des Ă©lĂ©ments majeurs des successions secondaires en rĂ©gion mĂ©diterranĂ©enne. Les Ă©tudes confirment les propriĂ©tĂ©s allĂ©lopathiques du pin d'Alep, propriĂ©tĂ©s qu'il faut nuancer en fonction des stades dynamiques et en fonction des sources d'allĂ©lochimiques (pluviolessivats ou exsudats racinaires). Enfin la mise en Ă©vidence de phĂ©nomĂšnes d'autotoxicitĂ© amĂšne une rĂ©flexion sur les rĂ©gulations de la dynamique populationnelle de ce pin et sur ses consĂ©quences sur la succession vĂ©gĂ©tale
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