6,130 research outputs found
Factoring Formal Maps into Reversible or Involutive Factors
An element of a group is called reversible if it is conjugate in the
group to its inverse. An element is an involution if it is equal to its
inverse. This paper is about factoring elements as products of reversibles in
the group of formal maps of , i.e.
formally-invertible -tuples of formal power series in variables, with
complex coefficients. The case was already understood.
Each product of reversibles has linear part of determinant .
The main results are that for each map with det is the
product of reversibles, and may also be factored as the product of
involutions, where is the smallest integer .Comment: 20 page
Forecasting the Number of Visitors in a Unique Recreational Site- A Retrospective View
We examine in the research forecasts prepared by us fifteen years ago. We examine the assumptions made as well as the results, comparing the forecasts to reality. We concentrate on the forecasts of number of visitors, which enables to examine economic impact, and is crucial in analyzing ecological carrying capacity. Our case study was a wetland that was drained in the '50s, resulting in severe environmental damages. In the '90s part of the area was re-flooded and a small lake was created. We forecasted the number of visitors, the expected revenues and benefits. The area is currently called Agmon Hula, located in the previous Hula marsh (North of Israel). The commodity planned was a site which offers safari, birds' sanctuary, horse riding, swimming in a pool, picnicking. We asked recreationists in adjacent national parks and nature reserves on their willingness to visit the planned park and their willingness to pay (WTP), using CVM methods. In reality, the site started operation in 2005 as a birds' sanctuary, due to its success in attracting birds. 500 million birds pass the area twice per year migrating to the south in the fall and returning north in the spring. Our forecast for 380 thousand visitors in the first year of operation did not materialize. We could have predicted a smaller number closer to the real number (220 thousand) if we would have considered the percentage that ranked birds' sanctuary as one of their two favorite activities. The prediction assumed an annual increase in the number of visitors of 2-6%, but actually, the increase in the first five years of operation is 8% annually. In the prediction, we disregarded tourists, but they were 7-17% of the visitors. Updating the prediction of number of visitors is easy, and is a crucial aspect in predicting carrying capacity.
Proximity Effect Enhancement Induced by Roughness of SN Interface
Critical temperature reduction is considered for a thin film of
a layered superconductor (S) with a rough surface covered by a thick layer of a
normal metal (N). The roughness of the SN interface increases the penetration
of electrons from the normal metal into the superconductor and leads to an
enhancement of the proximity effect. The value of induced by the
roughness of the SN interface can be much higher than for a film
with a plain surface for an extremely anisotropic layered superconductor with
the coherence lengths .Comment: 2 page
Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings
Granular packings slowly driven towards their instability threshold are
studied using a digital imaging technique as well as a nonlinear acoustic
method. The former method allows us to study grain rearrangements on the
surface during the tilting and the latter enables to selectively probe the
modifications of the weak-contact fraction in the material bulk. Gradual ageing
of both the surface activity and the weak-contact reconfigurations is observed
as a result of repeated tilt cycles up to a given angle smaller than the angle
of avalanche. For an aged configuration reached after several consecutive tilt
cycles, abrupt resumption of the on-surface activity and of the weak-contact
rearrangements occurs when the packing is subsequently inclined beyond the
previous maximal tilting angle. This behavior is compared with literature
results from numerical simulations of inclined 2D packings. It is also found
that the aged weak-contact configurations exhibit spontaneous restoration
towards the initial state if the packing remains at rest for tens of minutes.
When the packing is titled forth and back between zero and near-critical
angles, instead of ageing, the weak-contact configuration exhibits "internal
weak-contact avalanches" in the vicinity of both the near-critical and zero
angles. By contrast, the stronger-contact skeleton remains stable
A Discrete Version of the Inverse Scattering Problem and the J-matrix Method
The problem of the Hamiltonian matrix in the oscillator and Laguerre basis
construction from the S-matrix is treated in the context of the algebraic
analogue of the Marchenko method.Comment: 11 pages. The Laguerre basis case is adde
Collective Dynamics of One-Dimensional Charge Density Waves
The effect of disorder on the static and dynamic behaviour of one-dimensional
charge density waves at low temperatures is studied by analytical and numerical
approaches. In the low temperature region the spatial behaviour of the
phase-phase correlation function is dominated by disorder but the roughness
exponent remains the same as in the pure case. Contrary to high dimensional
systems the dependence of the creep velocity on the electric field is described
by an analytic function.Comment: 4 pages, 4 figure
Severe discrepancies between experiment and theory in the superconducting proximity effect
The superconducting proximity effect is investigated for SN double layers in
a regime where the resulting transition temperature T_{c} does not depend on
the mean free paths of the films and, within limits, not on the transparency of
the interface. This regime includes the thin film limit and the normalized
initial slope S_{sn}= (d_{s}/T_{s})|dT_{c}/dd_{n}|. The experimental results
for T_{c} are compared with a numerical simulation which was recently developed
in our group. The results for the SN double layers can be devided into three
groups: (i) When N = Cu, Ag, Au, Mg a disagreement between experiment and
theory by a factor of the order of three is observed, (ii) When N = Cd, Zn, Al
the disagreement between experiment and theory is reduced to a factor of about
1.5, (iii) When N = In, Sn a reasonably good agreement between experiment and
theory is observed
Quantitative Simulation of the Superconducting Proximity Effect
A numerical method is developed to calculate the transition temperature of
double or multi-layers consisting of films of super- and normal conductors. The
approach is based on a dynamic interpretation of Gorkov's linear gap equation
and is very flexible. The mean free path of the different metals, transmission
through the interface, ratio of specular reflection to diffusive scattering at
the surfaces, and fraction of diffusive scattering at the interface can be
included. Furthermore it is possible to vary the mean free path and the BCS
interaction NV in the vicinity of the interface. The numerical results show
that the normalized initial slope of an SN double layer is independent of
almost all film parameters except the ratio of the density of states. There are
only very few experimental investigations of this initial slope and they
consist of Pb/Nn double layers (Nn stands for a normal metal). Surprisingly the
coefficient of the initial slope in these experiments is of the order or less
than 2 while the (weak coupling) theory predicts a value of about 4.5. This
discrepancy has not been recognized in the past. The autor suggests that it is
due to strong coupling behavior of Pb in the double layers. The strong coupling
gap equation is evaluated in the thin film limit and yields the value of 1.6
for the coefficient. This agrees much better with the few experimental results
that are available.
PACS: 74.45.+r, 74.62.-c, 74.20.F
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