Revised spherically symmetric solutions of R+ε/RR+\varepsilon/R gravity

We study spherically symmetric static empty space solutions in $R+\varepsilon/R$ model of $f(R)$ gravity. We show that the Schwarzschild metric is an exact solution of the resulted field equations and consequently there are general solutions which {are perturbed Schwarzschild metric and viable for solar system. Our results for large scale contains a logarithmic term with a coefficient producing a repulsive gravity force which is in agreement with the positive acceleration of the universe.Comment: 8 page

Interacting viscous ghost tachyon, K-essence and dilaton scalar field models of dark energy

We study the correspondence between the interacting viscous ghost dark energy model with the tachyon, K-essence and dilaton scalar field models in the framework of Einstein gravity. We consider a spatially non-flat FRW universe filled with interacting viscous ghost dark energy and dark matter. We reconstruct both the dynamics and potential of these scalar field models according to the evolutionary behavior of the interacting viscous ghost dark energy model, which can describe the accelerated expansion of the universe. Our numerical results show that the interaction and viscosity have opposite effects on the evolutionary properties of the ghost scalar filed models.Comment: 16 pages, 17 figure

Reconstructing f(R) model from Holographic DE: Using the observational evidence

We investigate the corresponding relation between $f(R)$ gravity and an interacting holographic dark energy. By obtaining conditions needed for some observational evidence such as, positive acceleration expansion of universe, crossing the phantom divide line and validity of thermodynamics second law in an interacting HDE model and corresponding it with $f(R)$ mode of gravity we find a viable $f(R)$ model which can explain the present universe. We also obtain the explicit evolutionary forms of the corresponding scalar field, potential and scale factor of universe.Comment: 11page. phys. Scr (2012

L’Amélioration de la Sécurité du Chiffrement Algébrique Modulaire par les Générateurs de Fibonacci

Le Chiffre de Hill est l'un des algorithmes à clés symétriques qui a montré des avantages pour le cryptage de données. Cependant l'algorithme original est vulnérable aux «Attaque à Texte Clair Choisi». Un autre revers pour le Cryptage des images est qu’il révèle certaines tendances et ne cache pas toutes les caractéristiques de l'image (images avec des fonds homogènes). Pour surmonter ces problèmes, dans cet article, nous proposons une modification de l’algorithme de Hill en incluant une translation et en introduisant à chaque étape du processus du Cryptage des clés dynamiques Pseudo-Aléatoirement. La partie expérimentale de cet article prouve que la variante proposée donne un meilleur cryptage pour tous types d'images et montre bien une amélioration par rapport à l’algorithme conventionnel

Etude comparative de deux cryptosystèmes: L’AES versus l’attracteur Chaotique

Le transfert croissant de données multimédias engendre des besoins en termes de sécurité d’informations. Pour répondre aux normes de confidentialité, plusieurs algorithmes de cryptage ont été développés ces dernières années. En raison de l’utilisation massive des données images dans divers domaines (médicale, industrielle, …), il est nécessaire d’utiliser et d’adapter ces techniques de sécurité à ce type de données. Dans ce contexte, une étude comparative de deux algorithmes de cryptage appliqués aux images est réalisée. Le premier est le cryptosystème classique dit Advanced Encryption Standard (AES) et le deuxième issu des signaux chaotiques basés sur une carte logistique afin d’évaluer leurs robustesse en termes de sécurité

Modified gravity in a viscous and non-isotropic background

We study the dynamical evolution of an $f(R)$ model of gravity in a viscous and anisotropic background which is given by a Bianchi type-I model of the Universe. We find viable forms of $f(R)$ gravity in which one is exactly the Einsteinian model of gravity with a cosmological constant and other two are power law $f(R)$ models. We show that these two power law models are stable with a suitable choice of parameters. We also examine three potentials which exhibit the potential effect of $f(R)$ models in the context of scalar tensor theory. By solving different aspects of the model and finding the physical quantities in the Jordan frame, we show that the equation of state parameter satisfy the dominant energy condition. At last we show that the two power law $f(R)$ models behave like quintessence model at late times and also the shear coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure

Interacting New Agegraphic Dark Energy in a Cyclic Universe

The main goal of this work is investigation of NADE in the cyclic universe scenario. Since, cyclic universe is explained by a phantom phase ($\omega<-1$), it is shown when there is no interaction between matter and dark energy, ADE and NADE do not produce a phantom phase, then can not describe cyclic universe. Therefore, we study interacting models of ADE and NADE in the modified Friedmann equation. We find out that, in the high energy regime, which it is a necessary part of cyclic universe evolution, only NADE can describe this phantom phase era for cyclic universe. Considering deceleration parameter tells us that the universe has a deceleration phase after an acceleration phase, and NADE is able to produce a cyclic universe. Also it is found valuable to study generalized second law of thermodynamics. Since the loop quantum correction is taken account in high energy regime, it may not be suitable to use standard treatment of thermodynamics, so we turn our attention to the result of \citep{29}, which the authors have studied thermodynamics in loop quantum gravity, and we show that which condition can satisfy generalized second law of thermodynamics.Comment: 8 pages, 3 figure

Reconstruction of the equation of state for the cyclic universes in homogeneous and isotropic cosmology

We study the cosmological evolutions of the equation of state (EoS) for the universe in the homogeneous and isotropic Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) space-time. In particular, we reconstruct the cyclic universes by using the Weierstrass and Jacobian elliptic functions. It is explicitly illustrated that in several models the universe always stays in the non-phantom (quintessence) phase, whereas there also exist models in which the crossing of the phantom divide can be realized in the reconstructed cyclic universes.Comment: 29 pages, 8 figures, version accepted for publication in Central European Journal of Physic