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# SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

## 64 Couette experiment,

64 Couette experiment, Rayleigh-Btnard convection, and the Faraday experiment. This note was inspired by several conversations and seminars held at the week- long Workshop on Dynamics, Bifurcation, and Singularity Theory during the 1988-89 Warwick Symposium on Singularity Theory and its Applications. During the workshop it became clear that the same issues concerning boundary conditions were appearing in a variety of applications, and it was felt that a short note for the conference proceedings, exploring these issues, would be both appropriate and of general interest. We wish to thank the rather large group of participants, in particular Andrew Cliffe and Tom Mullin, who joined in those conversations and who helped formulate the ideas presented here. 1 Neumann Boundary Conditions and Symmetry As before, let 9 (u) denote a reaction-diffusion equation on the line, which is invariant under translations and reflections. Lemma 1.1 Solutions to \$>(u) = 0 satisfying Neumann boundary conditions on [0,x] are in 1:1 correspondence with solutions satisfying periodic boundary conditions on [-x,x] having the symmetry u(-x) = u(x). (1.1) Remark Def'me the two-element group B N = { 1,R} where R is the reflection Rx = -x. Then (1.1) consists of those functions fixed by B N. (1.2) (1.3) Proof We showed in the Introduction that solutions satisfying NBC lead to the desired type of solution satisfying PBC. It remains to prove the converse. Let u(x) be a smooth 2x-periodic solution to P(u) = 0 satisfying (1.1). Differentiating (1.1) implies tha'L u'(O) =0 and u'(-x)=-u'(x). Periodicity implies that u'(x) =-- u'(-x), so that u'(x) = O, whence u satisfies NBC on [0,~]. o As observed by Dangelmayr and Armbruster [1986, 1987], there are consequences of Lenmaa 1.1 for the genetic behaviour of bifurcations of PDEs with Neumann boundary conditions. This change stems from the fact that the bifurcation problem with PBC has 0(2) symmetry generated by translations modulo 2x and reflection. More precisely, the change in generic behaviour occurs as follows. Instead of studying the NBC problem, one first studies the PBC bifurcation problem using 0(2) symmetry, and then restricts the result to the fixed-point space FiX(BN) to recover the answer for NBC. The essential reason for the change in genericity is that the general O(2)-equivariant bifurcation problem obtained when analyzing the PBC case may not restrict to a general bifurcation problem on FiX(BN) with the symmetry of NBC. In cases where this restricted problem has special features we get a change in genericity.

65 We illustrate this point in the simplest instance, steady-state bifurcation. Assume that J~ depends on a bifurcation parameter ~, and that P has a trivial translation- invariant solution at ~. = 0 which, without loss of generality, we may assume to be u = 0. Finally, assume that this trivial solution undergoes a steacly-state bifucation at ~. = 0. Observe that the only symmetry of NBC is the reflection ~: x ~ ~-x. (1.4) Proposition 1.2 Under the above hypotheses on P (u) = 0, satisfying Neumann boundary conditions, we have: (a) Bifurcating solutions have a welbdefined non-negative integer mode number m. (b) Generically, when m > 0, the bifurcation is a pitchfork. Remarks (a) The mode number is associated with 'pattern' formation aJld can be observed in experiments. (b) For many operators the natural modes are obtained by separation of variables, leading to a spatial variation with eigenfunctions like cos(rex), For a general Z2--equivariant bifurcation it would be a surprise for this pure mode to occur as an eigenfunction. In general one would expect the eigenfunctions to be (perhaps inffmite) linear combinations ~ a k cos(kx). (e) The pitchfork bifurcation occurs for m even, even though when m is even (1.4) does not force this type of bifurcation in the NBC model. Proof By Lemma 1.1 there is a bifurcation at ~. = 0 in equilibrium solutions of \$~ (u) = 0 with PBC on [-re,x]. The group of symmetries of this trifurcation problem is 0(2). (a) Let L = d\$ ~ denote the linearized equations about u = 0 at k = 0 and let K = ker L. By 0(2) symmetry we expect K to be either 1- or 2-dimensional, since irreducible representations of 0(2) have those dimensions, Golubitsky, Stewart, and Schaeffer [1988] p. 330. We may write the action of SO(2) on K as z~ e im0 z, (1.5) where m = 0 in the simple eigenvalue case and m>0 in the double eigenvalue case. The integer m defined in (1.5) is the mode number. Let Z' denote the kernel of the n:presentation of 0(2) on K. Then 0(2) or SO(2) when m = 0 Z' = (1.6) Z m when m > 0 The isotropy subgroup Z of any bifurcating solution will contain Z'. Therefore bifurcating solutions will be translation..invariant when m = 0 (and hence constant), and invariant under translation by x ~ x + 2~x/m when m > 0. This translation-invariance is what gives the bifurcating solution a 'pattern'. When m > 0, Z is actually isomorphic to D m, generated by Z' and a reflection x ~ x0-x for some x 0.

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