arXiv subjects

G. Gaeta

Publications and source records attributed to G. Gaeta.

At least 37 records · Page 2Linked to original sources

arXiv2006

Asymptotic scaling symmetries for nonlinear PDEs

In some cases, solutions to nonlinear PDEs happen to be asymptotically (for large $x$ and/or $t$) invariant under a group $G$ which is not a symmetry of the equation. After recalling the geometrical meaning of symmetries of differential equations -- and solution-preserving maps -- we provide a precise definition of asymptotic symmetries of PDEs; we deal in particular, for ease of discussion and physical relevance, with scaling and translation symmetries of scalar equations. We apply the general discussion to a class of ``Richardson-like'' anomalous diffusion and reaction-diffusion equations, whose solution are known by numerical experiments to be asymptotically scale invariant; we obtain an analytical explanation of the numerically observed asymptotic scaling properties. We also apply our method to a different class of anomalous diffusion equations, relevant in optical lattices. The methods developed here can be applied to more general equations, as clear by their geometrical construction.

math-ph↗

arXiv2005

Finite group symmetry breaking

Finite group symmetry is commonplace in Physics, in particular through crystallographic groups occurring in condensed matter physics -- but also through the inversions (C,P,T and their combinations) occurring in high energy physics and field theory. The breaking of finite groups symmetry has thus been thoroughly studied, and general approaches exist to investigate it. In Landau theory, the state of a system is described by a finite dimensional variable (the {\it order parameter}), and physical states correspond to minima of a potential, invariant under a group. In this article we describe the basics of symmetry breaking analysis for systems described by a symmetric polynomial; in particular we discuss generic symmetry breakings, i.e. those determined by the symmetry properties themselves and independent on the details of the polynomial describing a concrete system. We also discuss how the plethora of invariant polynomials can be to some extent reduced by means of changes of coordinates, i.e. how one can reduce to consider certain types of polynomials with no loss of generality. Finally, we will give some indications on extension of this theory, i.e. on how one deals with symmetry breakings for more general groups and/or more general physical systems.

math-ph↗

arXiv2005

Asymptotic symmetries in an optical lattice

It was recently remarked by Lutz [{\it Phys. Rev. A} {\bf 67} (2003), 051402(R)] that the equation for the marginal Wigner distribution in an optical lattice admits a scale-free distribution corresponding to Tsallis statistics. Here we show that this distribution is invariant under an asymptotic symmetry of the equation, hence that this scale-free behavior can be understood in terms of symmetry analysis.

math-ph↗

arXiv2005

Asymptotic scaling in a model class of anomalous reaction-diffusion equations

We analyze asymptotic scaling properties of a model class of anomalous reaction-diffusion (ARD) equations. Numerical experiments show that solutions to these have, for large $t$, well defined scaling properties. We suggest a general framework to analyze asymptotic symmetry properties; this provides an analytical explanation of the observed asymptotic scaling properties for the considered ARD equations.

math-ph↗

arXiv2005

Asymptotic symmetries of difference equations on a lattice

It is known that many equations of interest in Mathematical Physics display solutions which are only asymptotically invariant under transformations (e.g. scaling and/or translations) which are not symmetries of the considered equation. In this note we extend the approach to asymptotic symmetries for the analysis of PDEs, to the case of difference equations.

math-ph↗

arXiv2005

Lambda and mu-symmetries

Lambda-symmetries of ODEs were introduced by Muriel and Romero, and discussed by C. Muriel in her talk at SPT2001. Here we provide a geometrical characterization of lambda-prolongations, and a generalization of these -- and of lambda-symmetries -- to PDEs and systems thereof.

math-ph↗

arXiv2005

Dimension increase and splitting for Poincare'-Dulac normal forms

Integration of nonlinear dynamical systems is usually seen as associated to a symmetry reduction, e.g. via momentum map. In Lax integrable systems, as pointed out by Kazhdan, Kostant and Sternberg in discussing the Calogero system, one proceeds in the opposite way, enlarging the nonlinear system to a system of greater dimension. We discuss how this approach is also fruitful in studying non integrable systems, focusing on systems in normal form.

math-ph↗

arXiv2004

On the geometry of lambda-symmetries, and PDEs reduction

We give a geometrical characterization of $λ$-prolongations of vector fields, and hence of $λ$-symmetries of ODEs. This allows an extension to the case of PDEs and systems of PDEs; in this context the central object is a horizontal one-form $μ$, and we speak of $μ$-prolongations of vector fields and $μ$-symmetries of PDEs. We show that these are as good as standard symmetries in providing symmetry reduction of PDEs and systems, and explicit invariant solutions.

math-ph↗

arXiv2004

Lie-Poincare' transformations and a reduction criterion in Landau theory

In the Landau theory of phase transitions one considers an effective potential $Φ$ whose symmetry group $G$ and degree $d$ depend on the system under consideration; generally speaking, $Φ$ is the most general $G$-invariant polynomial of degree $d$. When such a $Φ$ turns out to be too complicate for a direct analysis, it is essential to be able to drop unessential terms, i.e. to apply a simplifying criterion. Criteria based on singularity theory exist and have a rigorous foundation, but are often very difficult to apply in practice. Here we consider a simplifying criterion (as stated by Gufan) and rigorously justify it on the basis of classical Lie-Poincaré theory as far as one deals with fixed values of the control parameter(s) in the Landau potential; when one considers a range of values, in particular near a phase transition, the criterion has to be accordingly partially modified, as we discuss. We consider some specific cases of group $G$ as examples, and study in detail the application to the Sergienko-Gufan-Urazhdin model for highly piezoelectric perovskites.

math-ph↗

arXiv2004

Variational principles for involutive systems of vector fields

In many relevant cases -- e.g., in hamiltonian dynamics -- a given vector field can be characterized by means of a variational principle based on a one-form. We discuss how a vector field on a manifold can also be characterized in a similar way by means of an higher order variational principle, and how this extends to involutive systems of vector fields.

math-ph↗

arXiv2003

Maximal degree variational principles

Let $M$ be smooth $n$-dimensional manifold, fibered over a $k$-dimensional submanifold $B$ as $π:M \to B$, and $\vartheta \in Λ^k (M)$; one can consider the functional on sections $ϕ$ of the bundle $π$ defined by $\int_D ϕ^* (\vartheta)$, with $D$ a domain in $B$. We show that for $k = n-2$ the variational principle based on this functional identifies a unique (up to multiplication by a smooth function) nontrivial vector field in $M$, i.e. a system of ODEs. Conversely, any vector field $X$ on $M$ satisfying $i_X ({\rm d} \vartheta) = 0$ for some $\vartheta \in Λ^{n-2} (M)$ admits such a variational characterization. We consider the general case, and also the particular case $M = P \times R$ where one of the variables (the time) has a distinguished role; in this case our results imply that any Liouville (volume-preserving) vector field on the phase space $P$ admits a variational principle of the kind considered here.

math-ph↗

arXiv2003

Results and Limitations of the Soliton Theory of DNA Transcription

It has been suggested by several authors that nonlinear excitations, in particular solitary waves, could play a fundamental functional role in the process of DNA transcription, effecting the opening of the double chain needed for RNA Polymerase to be able to copy the genetic code. Some models have been proposed to model the relevant DNA dynamics in terms of a reduced number of effective degrees of freedom. Here I discuss advantages and disadvantages of such an approach, and discuss in more detail one of the models, i.e. the one proposed by Yakushevich.

q-bio.OT↗

arXiv2003

A variational principle for volume-preserving dynamics

We provide a variational description of any Liouville (i.e. volume preserving) autonomous vector fields on a smooth manifold. This is obtained via a ``maximal degree'' variational principle; critical sections for this are integral manifolds for the Liouville vector field. We work in coordinates and provide explicit formulae.

math-ph↗

arXiv2002

Quaternionic integrable systems

Standard (Arnold-Liouville) integrable systems are intimately related to complex rotations. One can define a generalization of these, sharing many of their properties, where complex rotations are replaced by quaternionic ones. Actually this extension is not limited to the integrable case: one can define a generalization of Hamilton dynamics based on hyperKahler structures.

math-ph↗

arXiv2002

Michel theory of symmetry breaking and gauge theories

We extend Michel's theorem on the geometry of symmetry breaking [L. Michel, {\it Comptes Rendus Acad. Sci. Paris} {\bf 272-A} (1971), 433-436] to the case of pure gauge theories, i.e. of gauge-invariant functionals defined on the space ${\cal C}$ of connections of a principal fiber bundle. Our proof follows closely the original one by Michel, using several known results on the geometry of ${\cal C}$. The result (and proof) is also extended to the case of gauge theories with matter fields.

math-ph↗

arXiv2002

Hyperhamiltonian dynamics

We introduce an extension of hamiltonian dynamics, defined on hyperkahler manifolds, which we call ``hyperhamiltonian dynamics''. We show that this has many of the attractive features of standard hamiltonian dynamics. We also discuss the prototypical integrable hyperhamiltonian systems, i.e. quaternionic oscillators.

math-ph↗

arXiv2001

Poincare' normal and renormalized forms

We briefly review the main aspects of (Poincaré-Dulac) normal forms; we have a look at the non-uniqueness problem, and discuss one of the proposed ways to ``further reduce'' the normal forms. We also mention some convergence results.

math-ph↗

arXiv2001

Linear Lie algebras with finite dimensional centralizers

We give criteria for finite dimensionality or infinite dimensionality of the polynomial centralizer of the Lie algebra of a linear Lie group, in terms of invariants and relative invariants of the group. In the finite dimensional scenario some applications to normal forms and to certain equations with fundamental solutions are presented.

math-ph↗