124 research outputs found
Twisting type-N vacuum fields with a group
We derive the equations corresponding to twisting type-N vacuum gravitational
fields with one Killing vector and one homothetic Killing vector by using the
same approach as that developed by one of us in order to treat the case with
two non-commuting Killing vectors. We study the case when the homothetic
parameter takes the value -1, which is shown to admit a reduction to a
third-order real ordinary differential equation for this problem, similar to
that previously obtained by one of us when two Killing vectors are present.Comment: LaTeX, 11 pages. To be published in Classical and Quantum Gravit
New first integral for twisting type-N vacuum gravitational fields with two non-commuting Killing vectors
A new first integral for the equations corresponding to twisting type-N
vacuum gravitational fields with two non-commuting Killing vectors is
introduced. A new reduction of the problem to a complex second-order ordinary
differential equation is given. Alternatively, the mentioned first integral can
be used in order to provide a first integral of the second-order complex
equation introduced in a previous treatment of the problem.Comment: 7 pages, LaTeX, uses ioplppt.sty and iopl12.sty; to be published in
Class. Quantum Gra
Gravitational fields with a non Abelian bidimensional Lie algebra of symmetries
Vacuum gravitational fields invariant for a bidimensional non Abelian Lie
algebra of Killing fields, are explicitly described. They are parameterized
either by solutions of a transcendental equation (the tortoise equation) or by
solutions of a linear second order differential equation on the plane.
Gravitational fields determined via the tortoise equation, are invariant for a
3-dimensional Lie algebra of Killing fields with bidimensional leaves. Global
gravitational fields out of local ones are also constructed.Comment: 8 pagese, latex, no figure
Convex Passivity Enforcement of Linear Macromodels via Alternate Subgradient Iterations
This paper introduces a new algorithm for passivity enforcement of linear lumped macromodels in scattering form. As typical in most state of the art passivity enforcement methods, we start with an initial non-passive macromodel obtained by a Vector Fitting process, and we perturb its parameters to make it passive. The proposed scheme is based on a convex formulation of both passivity constraints and objective function for accuracy preservation, thus allowing a formal proof of convergence to the unique optimal passive macromodel. This is a distinctive feature that differentiates the new scheme with respect to most state of the art methods, which either do not guarantee convergence or are not able to provide the most accurate solution. The presented algorithm can thus be safely used for those cases for which existing techniques fail. We illustrate the advantages of proposed method on a few benchmarks
The odd side of torsion geometry
We introduce and study a notion of `Sasaki with torsion structure' (ST) as an
odd-dimensional analogue of K\"ahler with torsion geometry (KT). These are
normal almost contact metric manifolds that admit a unique compatible
connection with 3-form torsion. Any odd-dimensional compact Lie group is shown
to admit such a structure; in this case the structure is left-invariant and has
closed torsion form.
We illustrate the relation between ST structures and other generalizations of
Sasaki geometry, and explain how some standard constructions in Sasaki geometry
can be adapted to this setting. In particular, we relate the ST structure to a
KT structure on the space of leaves, and show that both the cylinder and the
cone over an ST manifold are KT, although only the cylinder behaves well with
respect to closedness of the torsion form. Finally, we introduce a notion of
`G-moment map'. We provide criteria based on equivariant cohomology ensuring
the existence of these maps, and then apply them as a tool for reducing ST
structures.Comment: 34 pages; v2: added a small generalization (Proposition 3.6) of the
cone construction; two references added. To appear on Ann. Mat. Pura App
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
Exterior Differential System for Cosmological G2 Perfect Fluids and Geodesic Completeness
In this paper a new formalism based on exterior differential systems is
derived for perfect-fluid spacetimes endowed with an abelian orthogonally
transitive G2 group of motions acting on spacelike surfaces. This formulation
allows simplifications of Einstein equations and it can be applied for
different purposes. As an example a singularity-free metric is rederived in
this framework. A sufficient condition for a diagonal metric to be geodesically
complete is also provided.Comment: 27 pages, 0 figures, LaTeX2e, to be published in Classical and
Quantum Gravit
Riemannian submersions from almost contact metric manifolds
In this paper we obtain the structure equation of a contact-complex
Riemannian submersion and give some applications of this equation in the study
of almost cosymplectic manifolds with Kaehler fibres.Comment: Abh. Math. Semin. Univ. Hamb., to appea
La restauración de Sambucus palmensis en La Gomera: Conservación genética y modelización de nicho climático
La conservación de especies vegetales amenazadas a menudo conlleva la restauración de sus poblaciones naturales, ya sea mediante el refuerzo de las poblaciones existentes, la reintroducción de poblaciones extintas o la introducción de nuevos núcleos. Antes de comenzar acciones de restauración, es esencial conocer la biología de las especies con el fin de determinar los factores más importantes que limitan el crecimiento de la población fundadora (Heywood & Iriondo, 2003
- …