685 research outputs found
Comment on Ricci Collineations for type B warped space-times
We present two counterexamples to the paper by Carot et al. in Gen. Rel.
Grav. 1997, 29, 1223 and show that the results obtained are correct but not
general.Comment: LaTex, 3 pages, Eq. (9) and reference added, typos corrected; Gen.
Rel. Grav (to appear
On the general structure of Ricci collineations for type B warped spacetimes
A complete study of the structure of Ricci collineations for type B warped
spacetimes is carried out. This study can be used as a method to obtain these
symetries in such spacetimes. Special cases as 2+2 reducible spacetimes, and
plane and spherical symmetric spacetimes are considered specifically.Comment: 18 pages. Version accepted for publication in JM
Lie groups of conformal motions acting on null orbits
Space-times admitting a 3-dimensional Lie group of conformal motions
acting on null orbits are studied. Coordinate expressions for the metric and
the conformal Killing vectors (CKV) are provided (irrespectively of the matter
content) and then all possible perfect fluid solutions are found, although none
of these verify the weak and dominant energy conditions over the whole
space-time manifold.Comment: 5 pages, Late
La educación creadora de Arno Stern, el eco del dibujo infantil
Treball Final de Grau en Mestre o Mestra d'Educació Infantil. Codi: MI1040. Curs acadèmic: 2017/2018Que a todos los niños y niñas les entusiasme dibujar es un hecho. Desde pequeños coger un color
con la mano y que esta deje una huella en el papel les llama la atención. Todo lo que surge de
esta actividad se trata de un juego más. Pero cuando parece que ganen destreza, todo su
alrededor se interesa por interrumpir este juego “enseñándoles” a dibujar. Arno Stern quiso ir más
allá con todo esto, y de casualidad, se encontró con que había descubierto una nueva disciplina:
la Semiología de la expresión. Esta no sólo abandona la idea de intervenir en el “juego de pintar”,
como él lo denomina, sino que se replantea el origen de estos dibujos infantiles. Estos emanarán
de la memoria orgánica, memoria que recoge nuestras experiencias durante nuestra vida prenatal.
La manifestación y revelación de estos recuerdos, es posible gracias a la Formulación y la
evolución de sus estadios (Figuras Primarias, Objetos-Imágenes y Figuras Esenciales). Además
demuestra que todo esto ocurre de manera universal, independientemente del lugar, la cultura o
las condiciones climáticas, cada persona recurre para su expresión a las mismas fórmulasThat all children like drawing is a fact. Since they are small to take a colour with the hand and it
leaves a mark on the paper drawing their attention. Everything that comes up from this activity is
not more than a game. But when it seems that they gain dexterity, all their around is interested in
interrupting this game by "teaching" them how to draw. Arno Stern wanted to go further with all
this, and by chance, he found that he had discovered a new discipline: the Semiology of
expression. This not only abandons the idea of intervening in the "game of painting," as he calls it,
but also restates the origin of these children's drawings. These will emanate from the organic
memory, memory that collects our experiences during our prenatal life. The manifestation and the
revelation of these memories is possible thanks to the Formulation and the evolution of their states
(Primary Figures, Objects-Images and Essential Figures). It also demonstrates that all this
happens in a universal way, regardless of the place, the culture or the climatic conditions, each
person resorts for his expression to the same formula
The Einstein field equations for cylindrically symmetric elastic configurations
In the context of relativistic elasticity it is interesting to study axially symmetric space-times due to their significance in modeling neutron stars and other astrophysical systems of interest. To approach this problem, here, a particular class of these space-times is considered. A cylindrically symmetric elastic space-time configuration is studied, where the material metric is taken to be flat. The components of the energy-momentum tensor for elastic matter are written in terms of the invariants of the strain tensor, here chosen to be the eigenvalues of the pulled-back material metric. The Einstein field equations are presented and a condition confirming the existence of a constitutive function is obtained. This condition leads to special cases, in one of which a new system for the metric functions and an expression for the constitutive function are deduced. The new system depends on a particular function, which builds up the constitutive equation.IB and EGLRV acknowledge financial support from FCT and CMAT. They also express their thanks and gratitude for the hospitality at the Universitat de les Illes Balears
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