14,648 research outputs found

    Behavior and Breakdown of Higher-Order Fermi-Pasta-Ulam-Tsingou Recurrences

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    We investigate numerically the existence and stability of higher-order recurrences (HoRs), including super-recurrences, super-super-recurrences, etc., in the alpha and beta Fermi-Pasta-Ulam-Tsingou (FPUT) lattices for initial conditions in the fundamental normal mode. Our results represent a considerable extension of the pioneering work of Tuck and Menzel on super-recurrences. For fixed lattice sizes, we observe and study apparent singularities in the periods of these HoRs, speculated to be caused by nonlinear resonances. Interestingly, these singularities depend very sensitively on the initial energy and the respective nonlinear parameters. Furthermore, we compare the mechanisms by which the super-recurrences in the two model's breakdown as the initial energy and respective nonlinear parameters are increased. The breakdown of super-recurrences in the beta-FPUT lattice is associated with the destruction of the so-called metastable state and hence is associated with relaxation towards equilibrium. For the alpha-FPUT lattice, we find this is not the case and show that the super-recurrences break down while the lattice is still metastable. We close with comments on the generality of our results for different lattice sizes

    Rational approximations of f(R)f(R) cosmography through Pad\'e polynomials

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    We consider high-redshift f(R)f(R) cosmography adopting the technique of polynomial reconstruction. In lieu of considering Taylor treatments, which turn out to be non-predictive as soon as z>1z>1, we take into account the Pad\'e rational approximations which consist in performing expansions converging at high redshift domains. Particularly, our strategy is to reconstruct f(z)f(z) functions first, assuming the Ricci scalar to be invertible with respect to the redshift zz. Having the thus-obtained f(z)f(z) functions, we invert them and we easily obtain the corresponding f(R)f(R) terms. We minimize error propagation, assuming no errors upon redshift data. The treatment we follow naturally leads to evaluating curvature pressure, density and equation of state, characterizing the universe evolution at redshift much higher than standard cosmographic approaches. We therefore match these outcomes with small redshift constraints got by framing the f(R)f(R) cosmology through Taylor series around z≃0z\simeq 0. This gives rise to a calibration procedure with small redshift that enables the definitions of polynomial approximations up to z≃10z\simeq 10. Last but not least, we show discrepancies with the standard cosmological model which go towards an extension of the Λ\LambdaCDM paradigm, indicating an effective dark energy term evolving in time. We finally describe the evolution of our effective dark energy term by means of basic techniques of data mining.Comment: 11 pages, 14 figures, accepted for publication in JCA

    Role of cardiac resynchronization therapy in the development of new-onset atrial fibrillation: A single-center prospective study.

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    Albeit several studies examined the association between cardiac resynchronization therapy (CRT) and atrial fibrillation (AF) in heart failure (HF), results are still unclear and quite conflicting. We thereby designed a single-center prospective study to determine whether CRT has a favorable effect on the incidence of new-onset AF in a homogeneous population of patients with non-ischemic idiopathic dilated cardiomyopathy and severe heart failure HF. We enrolled 58 patients, AF naïve when received CRT. After 1 year of follow-up our population was subdivided into responders (72.4%) and non (27.6%), so to compare the incidence of AF after 1, 2 and 3 years of follow-up in these two groups. Already after 1 year, there is a significant (p<0.05) difference in new-onset AF in non-responder patients respect to responders (18.2% vs 3.3%). These data are confirmed at 2 year (33.3% vs 12.2%) and 3 year (50.0% vs 15.0%) follow-up. In particular, at 3 year follow-up, non-responders have an increased risk to develop new-onset AF (OR=5.67, 95% confidence interval = 1.36-23.59, p=0.019). The present work suggests a possible favorable role of this non-pharmacological therapy, on the prevention of AF

    Further stable neutron star models from f(R) gravity

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    Neutron star models in perturbative f(R)f(R) gravity are considered with realistic equations of state. In particular, we consider the FPS, SLy and other equations of state and a case of piecewise equation of state for stars with quark cores. The mass-radius relations for f(R)=R+R(e−R/R0−1)f(R)=R+R(e^{-R/R_{0}}-1) model and for R2R^2 models with logarithmic and cubic corrections are obtained. In the case of R2R^2 gravity with cubic corrections, we obtain that at high central densities (ρ>10ρns\rho>10\rho_{ns}, where ρns=2.7×1014\rho_{ns}=2.7\times 10^{14} g/cm3^{3} is the nuclear saturation density), stable star configurations exist. The minimal radius of such stars is close to 99 km with maximal mass ∼1.9M⊙\sim 1.9 M_{\odot} (SLy equation). A similar situation takes place for AP4 and BSK20 EoS. Such an effect can give rise to more compact stars than in General Relativity. If observationally identified, such objects could constitute a formidable signature for modified gravity at astrophysical level. Another interesting result can be achieved in modified gravity with only a cubic correction. For some EoS, the upper limit of neutron star mass increases and therefore these EoS can describe realistic star configurations (although, in General Relativity, these EoS are excluded by observational constraints).Comment: 18 pages, 17 figures, revised version significally expanded, to appear in JCA

    Maximal neutron star mass and the resolution of hyperon puzzle in modified gravity

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    The so-called hyperon puzzle in the theory of neutron stars is considered in the framework of modified f(R)f(R) gravity. We show that for simple hyperon equations of state, it is possible to obtain the maximal neutron star mass which satisfies the recent observational data for PSR J1614-2230, in higher-derivative models with power-law terms as f(R)=R+αR2+βR3f(R) = R+\alpha R^2+ \beta R^3. The soft hyperon equation of state under consideration is usually treated as non-realistic in the standard General Relativity. The numerical analysis of Mass-Radius relation for massive neutron stars with hyperon equation of state in modified gravity turns out to be consistent with observations. Thus, we show that the same modified gravity can solve at once three problems: consistent description of the maximal mass of neutron star, realistic Mass-Radius relation and account for hyperons in equation of state.Comment: 10 pages, 6 figures, some misprints are fixe

    Gauss-Bonnet dark energy by Lagrange multipliers

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    A string-inspired effective theory of gravity, containing Gauss-Bonnet invariant interacting with a scalar field, is considered in view of obtaining cosmological dark energy solutions. A Lagrange multiplier is inserted into the action in order to achieve the cosmological reconstruction by selecting suitable forms of couplings and potentials. Several cosmological exact solutions (including dark energy of quintessence, phantom or Little Rip type) are derived in presence and in absence of the Lagrange multiplier showing the difference in the two dynamical approaches. In the models that we consider, the Lagrange multiplier behaves as a sort of dust fluid that realizes the transitions between matter dominated and dark energy epochs. The relation between Lagrange multipliers and Noether symmetries is discussed.Comment: 14 pages, expanded version to appear in PR

    Dark energy from modified gravity with Lagrange multipliers

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    We study scalar-tensor theory, k-essence and modified gravity with Lagrange multiplier constraint which role is to reduce the number of degrees of freedom. Dark Energy cosmology of different types (Λ\LambdaCDM, unified inflation with DE, smooth non-phantom/phantom transition epoch) is reconstructed in such models. It is shown that mathematical equivalence between scalar theory and F(R)F(R) gravity is broken due to presence of constraint. The cosmological dynamics of F(R)F(R) gravity is modified by the second F2(R)F_2(R) function dictated by the constraint. Dark Energy cosmology is defined by this function while standard F1(R)F_1(R) function is relevant for local tests (modification of newton regime). A general discussion on the role of Lagrange multipliers to make higher-derivative gravity canonical is developed.Comment: LaTeX 12 pages, discussion is improve
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