14,648 research outputs found

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

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 $f(R)$ cosmography through Pad\'e polynomials

We consider high-redshift $f(R)$ cosmography adopting the technique of
polynomial reconstruction. In lieu of considering Taylor treatments, which turn
out to be non-predictive as soon as $z>1$, 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 $f(z)$
functions first, assuming the Ricci scalar to be invertible with respect to the
redshift $z$. Having the thus-obtained $f(z)$ functions, we invert them and we
easily obtain the corresponding $f(R)$ 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 $f(R)$ cosmology through Taylor series around $z\simeq 0$.
This gives rise to a calibration procedure with small redshift that enables the
definitions of polynomial approximations up to $z\simeq 10$. Last but not
least, we show discrepancies with the standard cosmological model which go
towards an extension of the $\Lambda$CDM 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.

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

Neutron star models in perturbative $f(R)$ 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 $f(R)=R+R(e^{-R/R_{0}}-1)$ model and
for $R^2$ models with logarithmic and cubic corrections are obtained. In the
case of $R^2$ gravity with cubic corrections, we obtain that at high central
densities ($\rho>10\rho_{ns}$, where $\rho_{ns}=2.7\times 10^{14}$ g/cm$^{3}$
is the nuclear saturation density), stable star configurations exist. The
minimal radius of such stars is close to $9$ km with maximal mass $\sim 1.9
M_{\odot}$ (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

The so-called hyperon puzzle in the theory of neutron stars is considered in
the framework of modified $f(R)$ 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 $f(R) = R+\alpha R^2+ \beta
R^3$. 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

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

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 ($\Lambda$CDM, 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
$F(R)$ gravity is broken due to presence of constraint. The cosmological
dynamics of $F(R)$ gravity is modified by the second $F_2(R)$ function dictated
by the constraint. Dark Energy cosmology is defined by this function while
standard $F_1(R)$ 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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