121880899
Reducible actions of D4⋉T2: superlattice patterns and hidden symmetries
We study steady-state pattern-forming instabilities on 2. A uniform initial state that is invariant under the Euclidean group E(2) of translations, rotations and reflections of the plane loses linear stability to perturbations with a non-zero wavenumber kc. We identify branches of solutions that are periodic on a square lattice that inherits a reducible action of the symmetry group D4T2. Reducible group actions occur naturally when we consider solutions that are periodic on real-space lattices that are much more widely spaced than the wavelength of the pattern-forming instability. They thus apply directly to computations in large domains where periodic boundary conditions are applied. The normal form for the bifurcation is calculated, taking the presence of various `hidden' symmetries into account and making use of previous work by Crawford [8]. We compute the stability (relative to other branches of solutions that exist on this lattice) of the solution branches that we can guarantee by applying the equivariant branching lemma. These computations involve terms higher than third order in the normal form, and are affected by the hidden symmetries. The effects of hidden symmetries that we elucidate are relevant also to bifurcations from fully nonlinear patterns. In addition, other primary branches of solutions with submaximal symmetry are found always to exist; their existence cannot be deduced by applying the equivariant branching lemma. These branches are stable in open regions of the space of normal form coefficients. The relevance of these results is illustrated by numerical simulations of a simple pattern-forming PDE.
123197869
Pattern formation outside of equilibrium
A comprehensive review of spatiotemporal pattern formation in systems driven away from equilibrium is presented, with emphasis on comparisons between theory and quantitative experiments. Examples include patterns in hydrodynamic systems such as thermal convection in pure fluids and binary mixtures, Taylor-Couette flow, parametric-wave instabilities, as well as patterns in solidification fronts, nonlinear optics, oscillatory chemical reactions and excitable biological media. The theoretical starting point is usually a set of deterministic equations of motion, typically in the form of nonlinear partial differential equations. These are sometimes supplemented by stochastic terms representing thermal or instrumental noise, but for macroscopic systems and carefully designed experiments the stochastic forces are often negligible. An aim of theory is to describe solutions of the deterministic equations that are likely to be reached starting from typical initial conditions and to persist at long times. A unified description is developed, based on the linear instabilities of a homogeneous state, which leads naturally to a classification of patterns in terms of the characteristic wave vector q0 and frequency ω0 of the instability. Type Is systems (ω0=0, q0≠0) are stationary in time and periodic in space; type IIIo systems (ω0≠0, q0=0) are periodic in time and uniform in space; and type Io systems (ω0≠0, q0≠0) are periodic in both space and time. Near a continuous (or supercritical) instability, the dynamics may be accurately described via "amplitude equations," whose form is universal for each type of instability. The specifics of each system enter only through the nonuniversal coefficients. Far from the instability threshold a different universal description known as the "phase equation" may be derived, but it is restricted to slow distortions of an ideal pattern. For many systems appropriate starting equations are either not known or too complicated to analyze conveniently. It is thus useful to introduce phenomenological order-parameter models, which lead to the correct amplitude equations near threshold, and which may be solved analytically or numerically in the nonlinear regime away from the instability. The above theoretical methods are useful in analyzing "real pattern effects" such as the influence of external boundaries, or the formation and dynamics of defects in ideal structures. An important element in nonequilibrium systems is the appearance of deterministic chaos. A greal deal is known about systems with a small number of degrees of freedom displaying "temporal chaos," where the structure of the phase space can be analyzed in detail. For spatially extended systems with many degrees of freedom, on the other hand, one is dealing with spatiotemporal chaos and appropriate methods of analysis need to be developed. In addition to the general features of nonequilibrium pattern formation discussed above, detailed reviews of theoretical and experimental work on many specific systems are presented. These include Rayleigh-Benard convection in a pure fluid, convection in binary-fluid mixtures, electrohydrodynamic convection in nematic liquid crystals, Taylor-Couette flow between rotating cylinders, parametric surface waves, patterns in certain open flow systems, oscillatory chemical reactions, static and dynamic patterns in biological media, crystallization fronts, and patterns in nonlinear optics. A concluding section summarizes what has and has not been accomplished, and attempts to assess the prospects for the future.
37367840
Planforms in two and three dimensions
SummaryWhen solving systems of PDE with two space dimensions it is often assumed that the solution is spatially doubly periodic. This assumption is usually made in systems such as the Boussinesq equation or reaction-diffusion equations where the equations have Euclidean invariance. In this article we use group theoretic techniques to determine a large class of spatially doubly periodic solutions that are forced to existence near a steady-state bifurcation from a translation-invariant equilibrium.This type of bifurcation problem has been considered by many authors when studying a number of different systems of PDE. Typically, these studies focus at the beginning on equilibria that are spatially periodic with respect to a fixed planar lattice type-such as square or hexagonal. Our focus is different in that we attempt to find all spatially periodic equilibria that bifurcate on all lattices. This point of view leads to some technical simplifications such as being able to restrict to translation free irreducible representations.Of course, many of the types of solutions that we find are well-known-such as hexagon and roll solutions on a hexagonal lattice. This coordinated group theoretic approach does lead, however, to solutions which seem not to have been discussed previously (antisquare solutions on a square lattice) as well as to a more complete classification of the symmetry types of possible solutions. Moreover, our methods extend to triply periodic solutions of PDE with three spatial variables. Some of these results, namely those concerned with primitive cubic lattices, are presented here. The complete results on triply periodic solutions may be found in [6, 7].
17335152
Stability Results for Steady , Spatially { Periodic
Equivariant bifurcation theory has been used extensively to study pattern formation via symmetry{breaking steady state bifurcation in various physical systems modeled by E(2){ equivariant partial diierential equations. Much attention has focussed on solutions that are doubly{periodic with respect to a square or hexagonal lattice, for which the bifurcation problem can be restricted to a nite{dimensional center manifold. Previous studies have used four{ and six{dimensional representations for the square and hexagonal lattice symmetry groups respectively, which in turn allows the relative stability of squares and rolls or hexagons and roll to be determined. Here we consider the countably innnite set of eight{ and twelve{ dimensional irreducible representations for the square and hexagonal cases, respectively. This extends earlier relative stability results to include a greater variety of bifurcating planforms, and also allows the stability of rolls, squares and hexagons to be established to a countably innnite set of perturbations. In each case we derive the Taylor expansion of the equivari-ant bifurcation problem and compute the linear, orbital stability of those solution branches guaranteed to exist by the equivariant branching lemma. In both cases we nd that many of the stability results are established at cubic order in the Taylor expansion, although to completely determine the stability of certain states, higher order terms are required. For the hexagonal lattice, all of the solution branches guaranteed by the equivariant branching lemma are, generically, unstable due to the presence of a quadratic term in the Taylor expansion. For this reason we consider two special cases: the degenerate bifurcation problem that is obtained by setting the coeecient of the quadratic term to zero, and the bifurcation problem when an extra reeection symmetry is present.
123668818
New planforms in systems of partial differential equations with Euclidean symmetry
Dionne & Golubitsky [10] consider the classification of planforms bifurcating (simultaneously) in scalar partial differential equations that are equivariant with respect to the Euclidean group in the plane. In particular, those planforms corresponding to isotropy subgroups with one-dimensional fixed-point space are classified.Many important Euclidean-equivariant systems of partial differential equations essentially reduce to a scalar partial differential equation, but this is not always true for general systems. We extend the classification of [10] obtaining precisely three planforms that can arise for general systems and do not exist for scalar partial differential equations. In particular, there is a class of one-dimensional ‘pseudoscalar’ partial differential equations for which the new planforms bifurcate in place of three of the standard planforms from scalar partial differential equations. For example, the usual rolls solutions are replaced by a nonstandard planform called anti-rolls. Scalar and pseudoscalar partial differential equations are distinguished by the representation of the Euclidean group.
124606689
A concise introduction to the theory of numbers
Preface Introduction 1. Divisibility 2. Arithmetical functions 3. Congruences 4. Quadratic residues 5. Quadratic forms 6. Diophantine approximation 7. Quadratic fields 8. Diophantine equations.
118504271
Applications of Centre Manifold Theory
0.- 4.5. The Case ?1 < 0.- 4.6. More Scaling.- 4.7. Completion of the Phase Portraits.- 4.8. Remarks and Exercises.- 4.9. Quadratic Nonlinearities.- 5. Application to a Panel Flutter Problem.- 5.1. Introduction.- 5.2. Reduction to a Second Order Equation.- 5.3. Calculation of Linear Terms.- 5.4. Calculation of the Nonlinear Terms.- 6. Infinite Dimensional Problems.- 6.1. Introduction.- 6.2. Semigroup Theory.- 6.3. Centre Manifolds.- 6.4. Examples.- References.
120138221
Classification and unfolding of degenerate Hopf bifurcations with O(2) symmetry: no distinguished parameter
Abstract The Hopf bifurcation in the presence of O(2) symmetry is considered. When the bifurcation breaks the symmetry, the critical imaginary eigenvalues have multiplicity two and generically there are two primary branches of periodic orbits which bifurcate simultaneously. In applications these correspond to rotating (traveling) waves and standing waves. Using equivariant singularity theory a classification of all such bifurcations up to and including codimension three is presented. No distinguished parameter is assumed. The universal unfoldings reveal the existence of both 2-tori and 3-tori; corresponding to quasiperiodic waves with two and three independent frequencies, respectively.
119748287
Bifurcation on the hexagonal lattice and the planar Bénard problem
One of the simplifications, used by Sattinger (1978), in studying the planar Bénard problem is to assume that the solutions are doubly periodic with respect to the hexagonal lattice in the plane. Once one makes this assumption, the generic situation is that the kernel of the linearized Boussinesq equations (linearized about the pure conduction solution) is six-dimensional, the eigenfunctions being superpositions of plane waves along three directions at mutual angles of 120°. In this situation the Liapunov-Schmidt procedure leads to a reduced bifurcation problem of the form g (x, λ) = 0 where g: [R6 x R -» R6 is smooth. Here λ represents the Rayleigh number. Moreover, such a g must commute with the symmetry group of the hexagonal lattice. In the paper we study such covariant bifurcation problems from the point of view of singularity theory and group theory, thus refining the work of Sattinger (1978). In particular we are able to classify the simplest such bifurcation problems as well as all of their perturbations. We find that stable rolls and stable hexagons occur as possible solutions. In addition, we find a rich structure of non-stable equilibrium solutions including wavy rolls and false hexagons appearing in the unfoldings of even the simplest degenerate bifurcation problems.
120280361
Toric geometry and equivariant bifurcations
Abstract Many problems in equivariant bifurcation theory involve the computation of invariant functions and equivariant mappings for the action of a torus group. We discuss general methods for finding these based on some elementary considerations related to toric geometry, a powerful technique in algebraic geometry. This approach leads to interesting combinatorial questions about cones in lattices, which lead to explicit calculations of minimal generating sets of invariants, from which the equivariants are easily deduced. We also describe the computation of Hilbert series for torus invariants and equivariants within the same combinatorial framework. As an example, we apply these methods to the interaction of two linear modes of a Euclidean-invariant PDE on a rectangular domain with periodic boundary conditions.
55668815
Compressible magnetoconvection in three dimensions: pattern formation in a strongly stratified layer
The interaction between magnetic fields and convection is interesting both because of its astrophysical importance and because the nonlinear Lorentz force leads to an especially rich variety of behaviour. We present several sets of computational results for magnetoconvection in a square box, with periodic lateral boundary conditions, that show transitions from steady convection with an ordered planform through a regime with intermittent bursts to complicated spatiotemporal behaviour. The constraints imposed by the square lattice are relaxed as the aspect ratio is increased. In wide boxes we find a new regime, in which regions with strong fields are separated from regions with vigorous convection. We show also how considerations of symmetry and associated group theory can be used to explain the nature of these transitions and the sequence in which the relevant bifurcations occur.
122803508
Nonlinear Competition between Small and Large Hexagonal Patterns
Recent experiments by Kudrolli, Pier and Gollub on surface waves, parametrically excited by two-frequency forcing, show a transition from a small hexagonal standing wave pattern to a triangular ``superlattice'' pattern. We show that generically the hexagons and the superlattice wave patterns bifurcate simultaneously from the flat surface state as the forcing amplitude is increased, and that the experimentally-observed transition can be described by considering a low-dimensional bifurcation problem. A number of predictions come out of this general analysis.
9719417
New Stability Results for Long { WavelengthConvection
We consider the transition from a spatially uniform state to a steady, spatially-periodic pattern in a partial diierential equation describing long-wavelength convec-tion 1]. This both extends existing work on the study of rolls, squares and hexagons and demonstrates how recent generic results for the stability of spatially-periodic patterns may be applied in practice. We nd that squares, even if stable to roll perturbations, are often unstable when a wider class of perturbations is considered. We also nd scenarios where transitions from hexagons to rectangles can occur. In some cases we nd that, near onset, more exotic spatially-periodic planforms are preferred over the usual rolls, squares and hexagons.