275 research outputs found
A Non - Singular Cosmological Model with Shear and Rotation
We have investigated a non-static and rotating model of the universe with an
imperfect fluid distribution. It is found that the model is free from
singularity and represents an ever expanding universe with shear and rotation
vanishing for large value of time.Comment: 10 pages, late
Phytosociological study of Hirschfeldia incana (L.) Lagraze-Fossat (Cruciferae) communities in mainland Greece
Using numerical analysis, the phytosociological study of Hirschfeldia incana communities in mainland Greece allowed their classification into the Rapistro rugosi-Hirschfeldietum incanae ass. nov., a new subnitrophilous association of the Hordeion leporini alliance. Three subassociations were distinguished (anthemidetosum incrassatae, hedypnoidetosum creticae and cardarietosum drabae), the distribution of which seems to depend on latitudinal alteration of rainfall. The new association has its optimum growth in habitats with moderate human influence, specifically in abandoned cultivations and wastelands. With respect to its floristic composition, the Rapistro rugosi-Hirschfeldietum incanae is close to anthropogenic vegetation with a high degree of naturalness, particularly to the therophytic,
subnitrophilous vegetation of the Thero-Brometalia (Stellarietea mediae) and the perennial, subnitrophilous vegetation of Carthametalia lanati (Artemisietea vulgaris)
Bianchi type II,III and V diagonal Einstein metrics re-visited
We present, for both minkowskian and euclidean signatures, short derivations
of the diagonal Einstein metrics for Bianchi type II, III and V. For the first
two cases we show the integrability of the geodesic flow while for the third
case a somewhat unusual bifurcation phenomenon takes place: for minkowskian
signature elliptic functions are essential in the metric while for euclidean
signature only elementary functions appear
Towards Canonical Quantum Gravity for G1 Geometries in 2+1 Dimensions with a Lambda--Term
The canonical analysis and subsequent quantization of the (2+1)-dimensional
action of pure gravity plus a cosmological constant term is considered, under
the assumption of the existence of one spacelike Killing vector field. The
proper imposition of the quantum analogues of the two linear (momentum)
constraints reduces an initial collection of state vectors, consisting of all
smooth functionals of the components (and/or their derivatives) of the spatial
metric, to particular scalar smooth functionals. The demand that the
midi-superspace metric (inferred from the kinetic part of the quadratic
(Hamiltonian) constraint) must define on the space of these states an induced
metric whose components are given in terms of the same states, which is made
possible through an appropriate re-normalization assumption, severely reduces
the possible state vectors to three unique (up to general coordinate
transformations) smooth scalar functionals. The quantum analogue of the
Hamiltonian constraint produces a Wheeler-DeWitt equation based on this reduced
manifold of states, which is completely integrated.Comment: Latex 2e source file, 25 pages, no figures, final version (accepted
in CQG
Further results on non-diagonal Bianchi type III vacuum metrics
We present the derivation, for these vacuum metrics, of the Painlev\'e VI
equation first obtained by Christodoulakis and Terzis, from the field equations
for both minkowskian and euclidean signatures. This allows a complete
discussion and the precise connection with some old results due to Kinnersley.
The hyperk\"ahler metrics are shown to belong to the Multi-Centre class and for
the cases exhibiting an integrable geodesic flow the relevant Killing tensors
are given. We conclude by the proof that for the Bianchi B family, excluding
type III, there are no hyperk\"ahler metrics.Comment: 21 pages, no figure
Essential Constants for Spatially Homogeneous Ricci-flat manifolds of dimension 4+1
The present work considers (4+1)-dimensional spatially homogeneous vacuum
cosmological models. Exact solutions -- some already existing in the
literature, and others believed to be new -- are exhibited. Some of them are
the most general for the corresponding Lie group with which each homogeneous
slice is endowed, and some others are quite general. The characterization
``general'' is given based on the counting of the essential constants, the
line-element of each model must contain; indeed, this is the basic contribution
of the work. We give two different ways of calculating the number of essential
constants for the simply transitive spatially homogeneous (4+1)-dimensional
models. The first uses the initial value theorem; the second uses, through
Peano's theorem, the so-called time-dependent automorphism inducing
diffeomorphismsComment: 26 Pages, 2 Tables, latex2
Words with the Maximum Number of Abelian Squares
An abelian square is the concatenation of two words that are anagrams of one
another. A word of length can contain distinct factors that
are abelian squares. We study infinite words such that the number of abelian
square factors of length grows quadratically with .Comment: To appear in the proceedings of WORDS 201
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