314 research outputs found
Investigating The Association Between Visual Perception Disorders And Difficulties In First Reading And Writing
Visual Perception is an important cognitive function or activity, through which the informational visual stimuli of the environment received by the sensory organs, are recognized as objects or events. The most important developmental period for the development of cognitive function of typical developed children is from 3.5 to 7.5 years, ie during their attendance in kindergarten and at the first grades of Primary School. This research, investigate the connection of Visual Perception Disorders with the appearance of possible difficulties in the first Reading and writing attempts.The tools of the research were the Detective Criterion of Visual Perception (AKOA 5-6) (KOUTRAS 2012) and the First Reading and Writing Test (VAMVOUKAS 2009). Moreover, the sample of the research were 300 preschool students. The analysis of the results recorded a statistically significant correlation between reading and visual and copying skills. As far as writing is concerned, it is statistically significant in relation to visual memory, visual discrimination of letter shapes and visual enclosure.
Interpreting a conformally flat pure radiation space-time
A physical interpretation is presented of the general class of conformally
flat pure radiation metrics that has recently been identified by Edgar and
Ludwig. It is shown that, at least in the weak field limit, successive wave
surfaces can be represented as null (half) hyperplanes rolled around a
two-dimensional null cone. In the impulsive limit, the solution reduces to a
pp-wave whose direction of propagation depends on retarded time. In the general
case, there is a coordinate singularity which corresponds to an envelope of the
wave surfaces. The global structure is discussed and a possible vacuum
extension through the envelope is proposed.Comment: 9 pages, Plain TeX, 2 figures. To appear in Class. Quantum Grav.
Reference adde
Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics, with zero cosmological constant
Metrics obtained by integrating within the generalised invariant formalism
are structured around their intrinsic coordinates, and this considerably
simplifies their invariant classification and symmetry analysis. We illustrate
this by presenting a simple and transparent complete invariant classification
of the conformally flat pure radiation metrics (except plane waves) in such
intrinsic coordinates; in particular we confirm that the three apparently
non-redundant functions of one variable are genuinely non-redundant, and easily
identify the subclasses which admit a Killing and/or a homothetic Killing
vector. Most of our results agree with the earlier classification carried out
by Skea in the different Koutras-McIntosh coordinates, which required much more
involved calculations; but there are some subtle differences. Therefore, we
also rework the classification in the Koutras-McIntosh coordinates, and by
paying attention to some of the subtleties involving arbitrary functions, we
are able to obtain complete agreement with the results obtained in intrinsic
coordinates. In particular, we have corrected and completed statements and
results by Edgar and Vickers, and by Skea, about the orders of Cartan
invariants at which particular information becomes available.Comment: Extended version of GRG publication, with some typos etc correcte
General non-rotating perfect-fluid solution with an abelian spacelike C_3 including only one isometry
The general solution for non-rotating perfect-fluid spacetimes admitting one
Killing vector and two conformal (non-isometric) Killing vectors spanning an
abelian three-dimensional conformal algebra (C_3) acting on spacelike
hypersurfaces is presented. It is of Petrov type D; some properties of the
family such as matter contents are given. This family turns out to be an
extension of a solution recently given in \cite{SeS} using completely different
methods. The family contains Friedman-Lema\^{\i}tre-Robertson-Walker particular
cases and could be useful as a test for the different FLRW perturbation
schemes. There are two very interesting limiting cases, one with a non-abelian
G_2 and another with an abelian G_2 acting non-orthogonally transitively on
spacelike surfaces and with the fluid velocity non-orthogonal to the group
orbits. No examples are known to the authors in these classes.Comment: Submitted to GRG, Latex fil
Self-similar Bianchi type VIII and IX models
It is shown that in transitively self-similar spatially homogeneous tilted
perfect fluid models the symmetry vector is not normal to the surfaces of
spatial homogeneity. A direct consequence of this result is that there are no
self-similar Bianchi VIII and IX tilted perfect fluid models. Furthermore the
most general Bianchi VIII and IX spacetime which admit a four dimensional group
of homotheties is given.Comment: 5 pages, Latex; One reference and minor clarifications added. To
appear in General Relativity and Gravitatio
On the Symmetries of the Edgar-Ludwig Metric
The conformal Killing equations for the most general (non-plane wave)
conformally flat pure radiation field are solved to find the conformal Killing
vectors. As expected fifteen independent conformal Killing vectors exist, but
in general the metric admits no Killing or homothetic vectors. However for
certain special cases a one-dimensional group of homotheties or motions may
exist and in one very special case, overlooked by previous investigators, a
two-dimensional homethety group exists. No higher dimensional groups of motions
or homotheties are admitted by these metrics.Comment: Plain TeX, 7 pages, No figure
On the limits of Brans-Dicke spacetimes: a coordinate-free approach
We investigate the limit of Brans-Dicke spacetimes as the scalar field
coupling constant omega tends to infinity applying a coordinate-free technique.
We obtain the limits of some known exact solutions. It is shown that these
limits may not correspond to similar solutions in the general relativity
theory.Comment: LaTeX, 16 pp, report DF/UFPB/02-9
Killing Tensors and Conformal Killing Tensors from Conformal Killing Vectors
Koutras has proposed some methods to construct reducible proper conformal
Killing tensors and Killing tensors (which are, in general, irreducible) when a
pair of orthogonal conformal Killing vectors exist in a given space. We give
the completely general result demonstrating that this severe restriction of
orthogonality is unnecessary. In addition we correct and extend some results
concerning Killing tensors constructed from a single conformal Killing vector.
A number of examples demonstrate how it is possible to construct a much larger
class of reducible proper conformal Killing tensors and Killing tensors than
permitted by the Koutras algorithms. In particular, by showing that all
conformal Killing tensors are reducible in conformally flat spaces, we have a
method of constructing all conformal Killing tensors (including all the Killing
tensors which will in general be irreducible) of conformally flat spaces using
their conformal Killing vectors.Comment: 18 pages References added. Comments and reference to 2-dim case.
Typos correcte
All spacetimes with vanishing curvature invariants
All Lorentzian spacetimes with vanishing invariants constructed from the
Riemann tensor and its covariant derivatives are determined. A subclass of the
Kundt spacetimes results and we display the corresponding metrics in local
coordinates. Some potential applications of these spacetimes are discussed.Comment: 24 page
Self-similar Bianchi models: I. Class A models
We present a study of Bianchi class A tilted cosmological models admitting a
proper homothetic vector field together with the restrictions, both at the
geometrical and dynamical level, imposed by the existence of the simply
transitive similarity group. The general solution of the symmetry equations and
the form of the homothetic vector field are given in terms of a set of
arbitrary integration constants. We apply the geometrical results for tilted
perfect fluids sources and give the general Bianchi II self-similar solution
and the form of the similarity vector field. In addition we show that
self-similar perfect fluid Bianchi VII models and irrotational Bianchi
VI models do not exist.Comment: 14 pages, Latex; to appear in Classical and Quantum Gravit
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