Página 1 dos resultados de 384 itens digitais encontrados em 0.005 segundos

## ON GENERIC ROTATIONLESS DIFFEOMORPHISMS OF THE ANNULUS

Fonte: AMER MATHEMATICAL SOC Publicador: AMER MATHEMATICAL SOC
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.03%
Let f be a C(r)-diffeomorphism of the closed annulus A that preserves the orientation, the boundary components and the Lebesgue measure. Suppose that f has a lift (f) over tilde to the infinite strip (A) over tilde which has zero Lebesgue measure rotation number. If the rotation number of f restricted to both boundary components of (f) over tilde is positive, then for such a generic f (r >= 16), zero is an interior point of its rotation set. This is a partial solution to a conjecture of P. Boyland.; CNPq[301485/03-8]; GNPq[304360/05-8]

## Quando as memórias são a matéria: memoriais de professoras alfabetizadoras e instabilidade genérica; When the memories are the matter: kindergarten teacher memorials and generic instability

Cális, Orasir Guilherme Teche
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Relevância na Pesquisa
66.56%
Tomando como ponto de partida a crítica às perspectivas que fazem prevalecer os aspectos estáveis dos gêneros discursivos em detrimento da fluidez que os relativiza, a presente pesquisa dá relevo aos elementos instáveis desse processo conjugando, a um só tempo, traços contínuos mais propensos à referida estabilização dos gêneros e descontínuos mais afeitos àquilo que chamo de instabilidade genérica. Do ponto de vista do processo de produção dos gêneros do discurso, a instabilidade genérica se inscreve no texto segundo diferentes marcas enunciativo-discursivas, assinalando pontos de ruptura e evidenciando um funcionamento marcado mais pela permanente incompletude do que pela pretensa fixidez de seus contornos. Parte-se da análise de corpus constituído por um conjunto de 84 textos produzidos, no ano de 2006, por professoras alfabetizadoras em um curso de formação (promovido pela Prefeitura Municipal de Cubatão-SP) chamado Letra e vida. Por ocasião desse curso, foi solicitado às participantes que redigissem, como atividade inicial, suas memórias de alfabetização, textos a partir dos quais elas deveriam reconstruir, preferencialmente de forma literária, as lembranças do tempo em que foram alfabetizadas. A partir dessa proposta...

## Gravitational instability on the brane: the role of boundary conditions

Shtanov, Yuri; Viznyuk, Alexander; Sahni, Varun
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.1%
An outstanding issue in braneworld theory concerns the setting up of proper boundary conditions for the brane-bulk system. Boundary conditions (BC's) employing regulatory branes or demanding that the bulk metric be nonsingular have yet to be implemented in full generality. In this paper, we take a different route and specify boundary conditions directly on the brane thereby arriving at a local and closed system of equations (on the brane). We consider a one-parameter family of boundary conditions involving the anisotropic stress of the projection of the bulk Weyl tensor on the brane and derive an exact system of equations describing scalar cosmological perturbations on a generic braneworld with induced gravity. Depending upon our choice of boundary conditions, perturbations on the brane either grow moderately (region of stability) or rapidly (instability). In the instability region, the evolution of perturbations usually depends upon the scale: small scale perturbations grow much more rapidly than those on larger scales. This instability is caused by a peculiar gravitational interaction between dark radiation and matter on the brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we find for the Dvali-Gabadadze-Porrati model an instability...

## On the stratorotational instability in the quasi-hydrostatic semi-geostrophic limit

Umurhan, O. M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.1%
The linear normal-mode stratorotational instability (SRI) is analytically reexamined in the inviscid limit where the length scales of horizontal disturbances are large compared their vertical and radial counterparts. Boundary conditions different than channel walls are also considered. This quasi-hydrostatic, semi-geostrophic (QHSG) approximation allows one to examine the effect of a vertically varying Brunt-Vaisaila frequency, $N^2$. It is found that the normal-mode instability persists when $N^2$ increases quadratically with respect to the disc vertical coordinate. However we also find that the SRI seems to exist in this inviscid QHSG extreme only for channel wall conditions: when one or both of the reflecting walls are removed there is no instability in the asymptotic limit explored here. It is also found that only exponential-type SRI modes (as defined by Dubrulle et al. 2005) exist under these conditions. These equations also admit non-normal mode behaviour. Fixed Lagrangian pressure conditions on both radial boundaries predicts there to be no normal mode behaviour in the QHSG limit. The mathematical relationship between the results obtained here and that of the classic Eady (1949) problem for baroclinic instability is drawn. We conjecture as to the mathematical/physical nature of the SRI. The general linear problem...

## Nonlinear supratransmission as a fundamental instability

Leon, J.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
35.94%
The nonlinear supratransmission is the property of a nonlinear system possessing a natural forbidden band gap to transmit energy of a signal with a frequency in the gap by means of generation of nonlinear modes (gap solitons). This process is shown to result from a generic instability of the evanescent wave profile generated in a nonlinear medium by the incident signal.; Comment: 4 figures, to appear in Physics Letters A

## The Instability of Charged Black Strings and p-Branes

Gregory, Ruth; Laflamme, Raymond
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.09%
We investigate the evolution of small perturbations around charged black strings and branes which are solutions of low energy string theory. We give the details of the analysis for the uncharged case which was summarized in a previous paper. We extend the analysis to the small charge case and give also an analysis for the generic case, following the behavior of unstable modes as the charge is modified. We study specifically a magnetically charged black 6-brane, but show how the instability is generic, and that charge does not in general stabilise black strings and p-branes.; Comment: 41 pages plain TeX, 6 figures appended at end of file, DAMTP/R-94/7,LA-UR-93-4473

## Resonance and marginal instability of switching systems

Protasov, Vladimir Y.; Jungers, Raphael M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.22%
We analyse the so-called Marginal Instability of linear switching systems, both in continuous and discrete time. This is a phenomenon of unboundedness of trajectories when the Lyapunov exponent is zero. We disprove two recent conjectures of Chitour, Mason, and Sigalotti (2012) stating that for generic systems, the resonance is sufficient for marginal instability and for polynomial growth of the trajectories. We provide a characterization of marginal instability under some mild assumptions on the sys- tem. These assumptions can be verified algorithmically and are believed to be generic. Finally, we analyze possible types of fastest asymptotic growth of trajectories. An example of a pair of matrices with sublinear growth is given.

## Gravitational Instability of de Sitter Compactifications

Contaldi, Carlo; Kofman, Lev; Peloso, Marco
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.09%
We consider warped compactifications in (4+d)-dimensional theories, with four dimensional de Sitter dS_4 vacua (with Hubble parameter H) and with a compact internal space. After introducing a gauge-invariant formalism for the generic metric perturbations of these backgrounds, we focus on modes which are scalar with respect to dS_4. The physical eigenmasses of these modes acquire a large universal tachyonic contribution -12d/(d+2) H^2, independently of the stabilization mechanism for the compact space, in addition to the usual KK masses, which instead encode the effects of the stabilization. General arguments, as well as specific examples, lead us to conjecture that, for sufficiently large dS curvature, the compactified geometry becomes gravitationally unstable due to the tachyonic growth of the scalar perturbations. This mean that for any stabilization mechanism the curvature of the dS geometry cannot exceed some critical value. We relate this effect to the anisotropy of the bulk geometry and suggest the end points of the instability. Of relevance for inflationary cosmology, the perturbations of the bulk metric inevitably induce a new modulus field, which describes the conformal fluctuations of the 4 dimensional metric. If this mode is light during inflation...

## Generic perturbations of linear integrable Hamiltonian systems

Bounemoura, Abed
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.36%
In this paper, we investigate perturbations of linear integrable Hamiltonian systems, with the aim of establishing results in the spirit of the KAM theorem (preservation of invariant tori), the Nekhoroshev theorem (stability of the action variables for a finite but long interval of time) and Arnold diffusion (instability of the action variables). Whether the frequency of the integrable system is resonant or not, it is known that the KAM theorem does not hold true for all perturbations; when the frequency is resonant, it is the Nekhoroshev theorem which does not hold true for all perturbations. Our first result deals with the resonant case: we prove a result of instability for a generic perturbation, which implies that the KAM and the Nekhoroshev theorem do not hold true even for a generic perturbation. The case where the frequency is non-resonant is more subtle. Our second result shows that for a generic perturbation, the KAM theorem holds true. Concerning the Nekhrosohev theorem, it is known that one has stability over an exponentially long interval of time, and that this cannot be improved for all perturbations. Our third result shows that for a generic perturbation, one has stability for a doubly exponentially long interval of time. The only question left unanswered is whether one has instability for a generic perturbation (necessarily after this very long interval of time).

## Semiclassical Instability of the Cauchy Horizon in Self-Similar Collapse

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.17%
Generic spherically symmetric self-similar collapse results in strong naked-singularity formation. In this paper we are concerned with particle creation during a naked-singularity formation in spherically symmetric self-similar collapse without specifying the collapsing matter. In the generic case, the power of particle emission is found to be proportional to the inverse square of the remaining time to the Cauchy horizon (CH). The constant of proportion can be arbitrarily large in the limit to marginally naked singularity. Therefore, the unbounded power is especially striking in the case that an event horizon is very close to the CH because the emitted energy can be arbitrarily large in spite of a cutoff expected from quantum gravity. Above results suggest the instability of the CH in spherically symmetric self-similar spacetime from quantum field theory and seem to support the existence of a semiclassical cosmic censor. The divergence of redshifts and blueshifts of emitted particles is found to cause the divergence of power to positive or negative infinity, depending on the coupling manner of scalar fields to gravity. On the other hand, it is found that there is a special class of self-similar spacetimes in which the semiclassical instability of the CH is not efficient. The analyses in this paper are based on the geometric optics approximation...

## Instability, Intermittency and Multiscaling in Discrete Growth Models of Kinetic Roughening

Dasgupta, C.; Kim, J. M.; Dutta, M.; Sarma, S. Das
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.12%
We show by numerical simulations that discretized versions of commonly studied continuum nonlinear growth equations (such as the Kardar-Parisi-Zhang equation and the Lai-Das Sarma equation) and related atomistic models of epitaxial growth have a generic instability in which isolated pillars (or grooves) on an otherwise flat interface grow in time when their height (or depth) exceeds a critical value. Depending on the details of the model, the instability found in the discretized version may or may not be present in the truly continuum growth equation, indicating that the behavior of discretized nonlinear growth equations may be very different from that of their continuum counterparts. This instability can be controlled either by the introduction of higher-order nonlinear terms with appropriate coefficients or by restricting the growth of pillars (or grooves) by other means. A number of such controlled instability'' models are studied by simulation. For appropriate choice of the parameters used for controlling the instability, these models exhibit intermittent behavior, characterized by multiexponent scaling of height fluctuations, over the time interval during which the instability is active. The behavior found in this regime is very similar to the turbulent'' behavior observed in recent simulations of several one- and two-dimensional atomistic models of epitaxial growth. [pacs{61.50.Cj...

## Instability driven by boundary inflow across shear: a way to circumvent Rayleigh's stability criterion in accretion disks?

Kerswell, R. R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.15%
We investigate the 2D instability recently discussed by Gallet et al. (2010) and Ilin \& Morgulis (2013) which arises when a radial crossflow is imposed on a centrifugally-stable swirling flow. By finding a simpler rectilinear example of the instability - a sheared half plane, the minimal ingredients for the instability are identified and the destabilizing/stabilizing effect of inflow/outflow boundaries clarified. The instability - christened boundary inflow instability' here - is of critical layer type where this layer is either at the inflow wall and the growth rate is $O(\sqrt{\eta})$ (as found by Ilin \& Morgulis 2013), or in the interior of the flow and the growth rate is $O(\eta \log 1/\eta)$ where $\eta$ measures the (small) inflow-to-tangential-flow ratio. The instability is robust to changes in the rotation profile even to those which are very Rayleigh-stable and the addition of further physics such as viscosity, 3-dimensionality and compressibility but is sensitive to the boundary condition imposed on the tangential velocity field at the inflow boundary. Providing the vorticity is not fixed at the inflow boundary, the instability seems generic and operates by the inflow advecting vorticity present at the boundary across the interior shear. Both the primary bifurcation to 2D states and secondary bifurcations to 3D states are found to be supercritical. Assuming an accretion flow driven by molecular viscosity only so $\eta=O(Re^{-1})$...

## Accurate simulations of the dynamical bar-mode instability in full General Relativity

Baiotti, Luca; De Pietri, Roberto; Manca, Gian Mario; Rezzolla, Luciano
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.16%
We present accurate simulations of the dynamical bar-mode instability in full General Relativity focussing on two aspects which have not been investigated in detail in the past. Namely, on the persistence of the bar deformation once the instability has reached its saturation and on the precise determination of the threshold for the onset of the instability in terms of the parameter $\beta={T}/{|W|}$. We find that generic nonlinear mode-coupling effects appear during the development of the instability and these can severely limit the persistence of the bar deformation and eventually suppress the instability. In addition, we observe the dynamics of the instability to be strongly influenced by the value $\beta$ and on its separation from the critical value $\beta_c$ marking the onset of the instability. We discuss the impact these results have on the detection of gravitational waves from this process and provide evidence that the classical perturbative analysis of the bar-mode instability for Newtonian and incompressible Maclaurin spheroids remains qualitatively valid and accurate also in full General Relativity.; Comment: RevTeX4, 23 pages, 19 figures. Version in print

## Onset of fingering instability in a finite slice of adsorbed solute

Hota, Tapan Kumar; Pramanik, Satayajit; Mishra, Manoranjan
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.12%
The effect of a linear adsorption isotherm on the onset of fingering instability in a miscible displacement in the application of liquid chromatography, pollutant contamination in aquifers etc. is investigated. Such fingering instability on the solute dynamics arise due to the miscible viscus fingering (VF) between the displacing fluid and sample solvent. We use a Fourier pseudo-spectral method to solve the initial value problem appeared in the linear stability analysis. The present linear stability analysis is of generic type and it captures the early time diffusion dominated region which was never expressible through the quasi-steady state analysis (QSSA). In addition, it measures the onset of instability more accurately than the QSSA methods. It is shown that the onset time depends non-monotonically on the retention parameter of the solute adsorption. This qualitative influence of the retention parameter on the onset of instability resemblances with the results obtained from direct numerical simulations of the nonlinear equations. Moreover, the present linear stability method helps for an appropriate characterisation of the linear and the nonlinear regimes of miscible VF instability and also can be useful for the fluid flow problems with the unsteady base-state.; Comment: 30 pages...

## Elliptical instability in terrestrial planets and moons

Cébron, David; Bars, Michael Le; Moutou, Claire; Gal, Patrice Le
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.31%
The presence of celestial companions means that any planet may be subject to three kinds of harmonic mechanical forcing: tides, precession/nutation, and libration. These forcings can generate flows in internal fluid layers, such as fluid cores and subsurface oceans, whose dynamics then significantly differ from solid body rotation. In particular, tides in non-synchronized bodies and libration in synchronized ones are known to be capable of exciting the so-called elliptical instability, i.e. a generic instability corresponding to the destabilization of two-dimensional flows with elliptical streamlines, leading to three-dimensional turbulence. We aim here at confirming the relevance of such an elliptical instability in terrestrial bodies by determining its growth rate, as well as its consequences on energy dissipation, on magnetic field induction, and on heat flux fluctuations on planetary scales. Previous studies and theoretical results for the elliptical instability are re-evaluated and extended to cope with an astrophysical context. In particular, generic analytical expressions of the elliptical instability growth rate are obtained using a local WKB approach, simultaneously considering for the first time (i) a local temperature gradient due to an imposed temperature contrast across the considered layer or to the presence of a volumic heat source and (ii) an imposed magnetic field along the rotation axis...

## Final State of Gregory-Laflamme Instability

Lehner, Luis; Pretorius, Frans
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.1%
We describe the behavior of a perturbed 5-dimensional black string subject to the Gregory-Laflamme instability. We show that the horizon evolves in a self-similar manner, where at any moment in the late-time development of the instability the horizon can be described as a sequence of 3-dimensional spherical black holes of varying size, joined by black string segments of similar radius. As with the initial black string, each local string segment is itself unstable, and this fuels the self-similar cascade to (classically) arbitrarily small scales; in the process the horizon develops a fractal structure. In finite asymptotic time, the remaining string segments shrink to zero-size, yielding a naked singularity. Since no fine-tuning is required to excite the instability, this constitutes a generic violation of cosmic censorship. We further discuss how this behavior is related to satellite formation in low-viscosity fluid streams subject to the Rayleigh-Plateau instability, and estimate the fractal dimension of the horizon prior to formation of the naked singularity.; Comment: 27 pages, 6 Figures. Chapter of the book Black Holes in Higher Dimensions' to be published by Cambridge University Press (editor: G. Horowitz)

## Pattern generation by dissipative parametric instability

Perego, A. M.; Tarasov, N.; Churkin, D. V.; Turitsyn, S. K.; Staliunas, K.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.12%
Nonlinear instabilities are responsible for spontaneous pattern formation in a vast number of natural and engineered systems ranging from biology to galaxies build-up. We propose a new instability mechanism leading to pattern formation in spatially extended nonlinear systems based on a periodic antiphase modulation of spectrally-dependent losses arranged in a zig-zag way: an effective filtering is imposed at symmetrically located wavenumbers k and -k in alternating order. The properties of the dissipative parametric instability differ from the features of the both key classical concepts of modulation instabilities: the Benjamin-Feir, and the Faraday instability. We demonstrate how dissipative parametric instability can lead to the formation of stable patterns in one and two-dimensional systems. The proposed instability mechanism is generic and can naturally occur or can be implemented in various physical systems.

## Libration driven elliptical instability

Cébron, David; Bars, Michael Le; Noir, J.; Aurnou, J. M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.1%
The elliptical instability is a generic instability which takes place in any rotating flow whose streamlines are elliptically deformed. Up to now, it has been widely studied in the case of a constant, non-zero differential rotation between the fluid and the elliptical distortion with applications in turbulence, aeronautics, planetology and astrophysics. In this letter, we extend previous analytical studies and report the first numerical and experimental evidence that elliptical instability can also be driven by libration, i.e. periodic oscillations of the differential rotation between the fluid and the elliptical distortion, with a zero mean value. Our results suggest that intermittent, space-filling turbulence due to this instability can exist in the liquid cores and sub-surface oceans of so-called synchronized planets and moons.

## Proof of linear instability of the Reissner-Nordstr\"om Cauchy horizon under scalar perturbations

Luk, Jonathan; Oh, Sung-Jin
Tipo: Artigo de Revista Científica
It has long been suggested that solutions to linear scalar wave equation $$\Box_g\phi=0$$ on a fixed subextremal Reissner-Nordstr\"om spacetime with non-vanishing charge are generically singular at the Cauchy horizon. We prove that generic smooth and compactly supported initial data on a Cauchy hypersurface indeed give rise to solutions with infinite nondegenerate energy near the Cauchy horizon in the interior of the black hole. In particular, the solution generically does not belong to $W^{1,2}_{loc}$. This instability is related to the celebrated blue shift effect in the interior of the black hole. The problem is motivated by the strong cosmic censorship conjecture and it is expected that for the full nonlinear Einstein-Maxwell system, this instability leads to a singular Cauchy horizon for generic small perturbations of Reissner-Nordstr\"om spacetime. Moreover, in addition to the instability result, we also show as a consequence of the proof that Price's law decay is generically sharp along the event horizon.