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 $C_3$ 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