270 research outputs found
The symmetric, D-invariant and Egorov reductions of the quadrilateral lattice
We present a detailed study of the geometric and algebraic properties of the
multidimensional quadrilateral lattice (a lattice whose elementary
quadrilaterals are planar; the discrete analogue of a conjugate net) and of its
basic reductions. To make this study, we introduce the notions of forward and
backward data, which allow us to give a geometric meaning to the tau-function
of the lattice, defined as the potential connecting these data. Together with
the known circular lattice (a lattice whose elementary quadrilaterals can be
inscribed in circles; the discrete analogue of an orthogonal conjugate net) we
introduce and study two other basic reductions of the quadrilateral lattice:
the symmetric lattice, for which the forward and backward data coincide, and
the D-invariant lattice, characterized by the invariance of a certain natural
frame along the main diagonal. We finally discuss the Egorov lattice, which is,
at the same time, symmetric, circular and D-invariant. The integrability
properties of all these lattices are established using geometric, algebraic and
analytic means; in particular we present a D-bar formalism to construct large
classes of such lattices. We also discuss quadrilateral hyperplane lattices and
the interplay between quadrilateral point and hyperplane lattices in all the
above reductions.Comment: 48 pages, 6 figures; 1 section added, to appear in J. Geom. & Phy
On the occurrence of gauge-dependent secularities in nonlinear gravitational waves
We study the plane (not necessarily monochromatic) gravitational waves at
nonlinear quadratic order on a flat background in vacuum. We show that, in the
harmonic gauge, the nonlinear waves are unstable. We argue that, at this order,
this instability can not be eliminated by means of a multiscale approach, i.e.
introducing suitable long variables, as it is often the case when secularities
appear in a perturbative scheme. However, this is a non-physical and
gauge-dependent effect that disappears in a suitable system of coordinates. In
facts, we show that in a specific gauge such instability does not occur, and
that it is possible to solve exactly the second order nonlinear equations of
gravitational waves. Incidentally, we note that this gauge coincides with the
one used by Belinski and Zakharov to find exact solitonic solutions of
Einstein's equations, that is to an exactly integrable case, and this fact
makes our second order nonlinear solutions less interesting. However, the
important warning is that one must be aware of the existence of the instability
reported in this paper, when studying nonlinear gravitational waves in the
harmonic gauge
The self-adjoint 5-point and 7-point difference operators, the associated Dirichlet problems, Darboux transformations and Lelieuvre formulas
We present some basic properties of two distinguished discretizations of
elliptic operators: the self-adjoint 5-point and 7-point schemes on a two
dimensional lattice. We first show that they allow to solve Dirichlet boundary
value problems; then we present their Darboux transformations. Finally we
construct their Lelieuvre formulas and we show that, at the level of the normal
vector and in full analogy with their continuous counterparts, the self-adjoint
5-point scheme characterizes a two dimensional quadrilateral lattice (a lattice
whose elementary quadrilaterals are planar), while the self-adjoint 7-point
scheme characterizes a generic 2D lattice.Comment: 20 pages, 6 figures, submitted to Glasgow Mathematical Journal Trust
for Island II proceedind
Integrable dynamics of a discrete curve and the Ablowitz-Ladik hierarchy
We show that the following elementary geometric properties of the motion of a
discrete (i.e. piecewise linear) curve select the integrable dynamics of the
Ablowitz-Ladik hierarchy of evolution equations: i) the set of points
describing the discrete curve lie on the sphere S^3, ii) the distance between
any two subsequant points does not vary in time, iii) the dynamics does not
depend explicitly on the radius of the sphere. These results generalize to a
discrete context our previous work on continuous curves.Comment: LaTeX file, 14 pages + 4 figure
Integrable Discrete Geometry: the Quadrilateral Lattice, its Transformations and Reductions
We review recent results on Integrable Discrete Geometry. It turns out that
most of the known (continuous and/or discrete) integrable systems are
particular symmetries of the quadrilateral lattice, a multidimensional lattice
characterized by the planarity of its elementary quadrilaterals. Therefore the
linear property of planarity seems to be a basic geometric property underlying
integrability. We present the geometric meaning of its tau-function, as the
potential connecting its forward and backward data. We present the theory of
transformations of the quadrilateral lattice, which is based on the discrete
analogue of the theory of rectilinear congruences. In particular, we discuss
the discrete analogues of the Laplace, Combescure, Levy, radial and fundamental
transformations and their interrelations. We also show how the sequence of
Laplace transformations of a quadrilateral surface is described by the discrete
Toda system. We finally show that these classical transformations are strictly
related to the basic operators associated with the quantum field theoretical
formulation of the multicomponent Kadomtsev-Petviashvilii hierarchy. We review
the properties of quadrilateral hyperplane lattices, which play an interesting
role in the reduction theory, when the introduction of additional geometric
structures allows to establish a connection between point and hyperplane
lattices. We present and fully characterize some geometrically distinguished
reductions of the quadrilateral lattice, like the symmetric, circular and
Egorov lattices; we review also basic geometric results of the theory of
quadrilateral lattices in quadrics, and the corresponding analogue of the
Ribaucour reduction of the fundamental transformation.Comment: 27 pages, 9 figures, to appear in Proceedings from the Conference
"Symmetries and Integrability of Difference Equations III", Sabaudia, 199
Evidence for detrimental cross interactions between reactive oxygen and nitrogen species in Leber's hereditary optic neuropathy cells
Here we have collected evidence suggesting that chronic changes in the NO homeostasis and the rise of reactive oxygen species
bioavailability can contribute to cell dysfunction in Leber’s hereditary optic neuropathy (LHON) patients.We report that peripheral
blood mononuclear cells (PBMCs), derived froma female LHON patient with bilateral reduced vision and carrying the pathogenic
mutation 11778/ND4, display increased levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS), as revealed
by flow cytometry, fluorometric measurements of nitrite/nitrate, and 3-nitrotyrosine immunodetection. Moreover, viability assays
with the tetrazolium dye MTT showed that lymphoblasts from the same patient are more sensitive to prolonged NO exposure,
leading to cell death. Taken together these findings suggest that oxidative and nitrosative stress cooperatively play an important
role in driving LHON pathology when excess NO remains available over time in the cell environment
The nexus between forest fragmentation in Africa and Ebola virus disease outbreaks
Tropical forests are undergoing land use change in many regions of the world, including the African continent. Human populations living close to forest margins fragmented and disturbed by deforestation may be particularly exposed to zoonotic infections because of the higher likelihood for humans to be in contact with disease reservoirs. Quantitative analysis of the nexus between deforestation and the emergence of Ebola virus disease (EVD), however, is still missing. Here we use land cover change data in conjunction with EVD outbreak records to investigate the association between recent (2004-2014) outbreaks in West and Central Africa, and patterns of land use change in the region. We show how in these EVD outbreaks the index cases in humans (i.e. spillover from wildlife reservoirs) occurred mostly in hotspots of forest fragmentation
Cooperative Intersection Crossing Over 5G
IEEE Autonomous driving is a safety critical application of sensing and decision-making technologies. Communication technologies extend the awareness capabilities of vehicles, beyond what is achievable with the on-board systems only. Nonetheless, issues typically related to wireless networking must be taken into account when designing safe and reliable autonomous systems. The aim of this work is to present a control algorithm and a communication paradigm over 5G networks for negotiating traffic junctions in urban areas. The proposed control framework has been shown to converge in a finite time and the supporting communication software has been designed with the objective of minimizing communication delays. At the same time, the underlying network guarantees reliability of the communication. The proposed framework has been successfully deployed and tested, in partnership with Ericsson AB, at the AstaZero proving ground in Goteborg, Sweden. In our experiments, three heterogeneous autonomous vehicles successfully drove through a 4-way intersection of 235 square meters in an urban scenario
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