Equivariant dynamical system ($F(gx,\mu )=gF(x,\mu )$)

An equivariant dynamical system has a vector field commuting with the action of a symmetry group. Its flow map preserves every subgroup's fixed-point subspace, because equivariance and uniqueness keep initial states fixed by that subgroup. Polynomial normal forms are constrained by the weights and parity of the group representation.

If a translation symmetry acts on complex Fourier modes by $A_j\mapsto e^{i{q}_j\cdot s}{A}_j$, a linear combination of their phases is invariant precisely when its coefficient vector lies in the kernel of the matrix of wavevectors. For four amplitudes whose wavevectors span a plane, this leaves two independent phase variables after quotienting translations. At zero amplitude, polar phases are undefined, so smooth centre manifold equations should first be written in Cartesian amplitudes.

An equivariant dynamical system can contain the monomial ${\prod }_j{A}_j^{p_j}{\overline{A}}_j^{q_j}$ in its $\ell$th amplitude equation only if its total wavevector equals the output wavevector. Its degree is ${\sum }_j(p_j+q_j)$. Thus an integer vector $d=p-q$ contributes at degree $N$ exactly when the wavevector equation holds, $\Vert d{\Vert }_1\le N$, and $N-\Vert d{\Vert }_1$ is even. The extra even degree is supplied by factors $|A_j{|}^2$. This finite integer test distinguishes regular saturation terms from phase-sensitive resonance terms.

Independent coordinate reflections require the $i$th component to be odd in $x_i$ and even in every other coordinate. Permutations force the same two cubic coefficients in all components. Absolute irreducibility forces a scalar linear coefficient, which can be normalized to the transverse bifurcation parameter. No quadratic terms occur. The axial branches with one, two or three equal nonzero absolute coordinates have leading squared amplitude $-\mu /[a+(m-1)b]$, provided that denominator is nonzero.

At a nondegenerate equivariant Hopf bifurcation, each complex axial isotropy subgroup of the spatial group combined with temporal phase gives a branch of periodic solutions with that spatiotemporal symmetry. Assumptions include an isolated semisimple imaginary critical pair of the required representation type, transverse parameter crossing, and a nonzero cubic coefficient on the fixed-point plane. The statement guarantees existence and symmetry type, not stability or which parameter side contains every branch.

A primary periodic branch whose phase advance is compensated by a spatial rotation. For the natural plane representation, the spatial pattern rotates rather than remaining on a fixed reflection axis. Opposite chiralities are exchanged by reflection.

// Target: dynamical-systems.bigb

A primary periodic branch retaining a spatial reflection at every time. In the natural plane representation, each real-copy component oscillates along a fixed reflection axis. The fourfold dihedral case has distinct axial and diagonal standing-wave types.

// Target: dynamical-systems.bigb

In Hopf coordinates where the rotation acts as $(i{q}_1,-i{q}_2)$, reflection swaps components, and temporal phase multiplies both equally, cubic equivariance permits precisely the three displayed monomials in the first component. Swapping indices gives the second equation, with the same complex coefficients. The rotating, axial standing, and diagonal standing branches are $q_2=0$, $q_2=q_1$, and $q_2=i{q}_1$. Their squared component amplitudes are $\mu /\mathrm{Re}a$, $\mu /\mathrm{Re}(a+b+c)$, and $\mu /\mathrm{Re}(a+b-c)$, respectively, when positive.

// Target: dynamical-systems.bigb

Spatially complex coordinates on two real copies need not both have the same temporal phase weight. If their time-circle action is $(z_1,z_2)\mapsto (e^{-i\varphi }{z}_1,e^{i\varphi }{z}_2)$, conjugating the second coordinate gives a common Hopf phase. The linear terms then have the same imaginary frequency in $q$ coordinates but opposite imaginary frequencies in the original $z$ coordinates. Failing to make this conjugation produces amplitude equations inconsistent with the specified symmetries.

// Target: dynamical-systems.bigb

A spatial group element combined with a time shift may preserve a periodic orbit even if the spatial element alone does not fix it at each instant. The temporal circle acts complex linearly on Hopf amplitudes. Complex axial isotropy subgroups identify the primary branches guaranteed by the equivariant Hopf theorem.

// Target: dynamical-systems.bigb

For the $D_3$ action $\rho (z_1,z_2)=({\zeta }^{-1}{z}_1,\zeta z_2)$, $m(z_1,z_2)=(z_2,z_1)$ and common phase multiplication, the fifth-order normal form has first component $z_1[\alpha +a|z_1{|}^2+b|z_2{|}^2+c|z_1{|}^4+d|z_1{|}^2|z_2{|}^2+e|z_2{|}^4]+f{\bar{z}}_1^2{z}_2^3$ and second component obtained by swapping indices. Here all coefficients may be complex. Its three primary symmetry types are a one-component rotating wave and the two standing-wave lines $z_1=z_2$ and $z_1=-z_2$, together with their group conjugates. The two standing amplitudes split at fifth order through $\mathrm{Re}f$.

If only reflection and common temporal phase remain, the linear complex matrix is $\left(\begin{array}{cc}\alpha & \eta \\ \eta & \alpha \end{array}\right)$. The symmetric and antisymmetric modes have eigenvalues $\alpha +\eta$ and $\alpha -\eta$, generically separating their Hopf thresholds. One-component rotating-wave axes are no longer invariant. Their hyperbolic finite-amplitude periodic solutions can persist as mixed-mode solutions, often through secondary symmetry-breaking bifurcations, while the double primary onset splits into two simple Hopf onsets. Coefficients determine the detailed secondary diagram and stability.

For an absolutely irreducible real symmetry representation, an equivariant steady-state bifurcation with a simple transverse crossing of the scalar linear eigenvalue generically has an equilibrium branch for each axial isotropy type. Restricting the vector field to its one-dimensional fixed-point subspace reduces the existence problem to a scalar bifurcation equation. The lemma guarantees these branches; it does not exclude branches with smaller isotropy or higher-dimensional fixed-point subspaces.

The normalizer of an axial isotropy subgroup preserves its one-dimensional fixed-point subspace of a group action. If its quotient by the isotropy contains an element acting as minus one, the reduced scalar vector field is odd, forcing a pitchfork bifurcation rather than a generic quadratic branch. For odd dihedral groups the reflection normalizer has no such quotient; an even-degree anisotropic term can distinguish the two half-axes at higher order.

// Target: algebra.bigb

For the natural plane action of the dihedral group, rotations constrain monomial weights and reflection makes coefficients real. The reflection axes support the bifurcating equilibria. For $n=3$, the quadratic anisotropic term gives three transcritical-like branches that are saddles in the full plane. For $n=4$, cubic isotropic and anisotropic terms determine two reflection-isotropy classes. For $n=5$, the cubic isotropic term sets the leading radial amplitude while quartic anisotropy separates stable and saddle angular classes.

// Target: dynamical-systems.bigb

Independent sign changes and interchange of two coordinates in the natural dihedral group action force the displayed cubic normal form. Apart from the origin, its nondegenerate small equilibria lie on coordinate axes or on the diagonals. An axial branch has squared amplitude $\mu /a$ and eigenvalues $-2\mu ,\mu (1-b/a)$. A diagonal branch has component squared amplitude $\mu /(a+b)$ and eigenvalues $-2\mu ,2\mu (b-a)/(a+b)$. Subtracting the two nonzero equilibrium equations gives $(a-b)(A^2-B^2)=0$, excluding other cubic branches when $a\ne b$.

For the four wavevectors $(2,1),(2,-1),(1,2),(1,-2)$ and their negatives, use invariant phases ${\chi }_1=2{\theta }_1-2{\theta }_2-{\theta }_3+{\theta }_4$, ${\chi }_2={\theta }_1+{\theta }_2-2{\theta }_3-2{\theta }_4$. Add to the cubic normal form of a dynamical system the two quintic terms $e{\overline{A}}_2{A}_3^2{A}_4^2$ and $f{\overline{A}}_1{A}_2^2{A}_3{\overline{A}}_4$ in the first equation, together with their square-symmetry partners; $e,f$ are real. For equal positive magnitudes, steadiness forces $\sin {\chi }_1=\sin {\chi }_2=0$ unless $e=f=0$. Equality of radial equations further gives $(e-f)(\cos {\chi }_1-\cos {\chi }_2)=0$. Consequently the generic phase types are $(0,0)$ and $(\pi ,\pi )$. When $e=f\ne 0$, the mixed type $(0,\pi )$, equivalent by square symmetry to $(\pi ,0)$, also exists. When both quintic coefficients vanish, every phase pair is allowed. Amplitude existence must still be checked from the remaining scalar equation.

A scalar mode that changes sign under a quarter-turn and is fixed by diagonal reflections couples quadratically through $AC,BC,A^2-B^2$. In the saturated three-mode equivariant dynamical system, a diagonal state $A=B=s$, $C=0$ has $s^2={\mu }_1/(1+\lambda )$ and $p=2(\lambda -1)s^2$. The symmetric perturbation decays at rate $2{\mu }_1$; the remaining stability matrix is the displayed block. With $K=2{\alpha }_1{\gamma }_1/(\lambda -1)>0$, its linear stability interval is $-K<{\mu }_2<-p$, provided $p<K$. The endpoints are a symmetry-breaking pitchfork bifurcation and a Hopf bifurcation, respectively, away from degeneracies.