A Bifurcation Lemma for Invariant Subspaces

The bifurcation from a simple eigenvalue (BSE) theorem is the foundation of steady-state bifurcation theory for one-parameter families of functions. When eigenvalues of multiplicity greater than one are caused by symmetry, the equivariant branching lemma (EBL) can often be applied to predict the branching of solutions. The EBL can be interpreted as the application of the BSE theorem to a fixed point subspace. There are functions which have invariant linear subspaces that are not caused by symmetry. For example, networks of identical coupled cells often have such invariant subspaces. We present a generalization of the EBL, where the BSE theorem is applied to nested invariant subspaces. We call this the bifurcation lemma for invariant subspaces (BLIS). We give several examples of bifurcations and determine if BSE, EBL, or BLIS applies. We extend our previous automated bifurcation analysis algorithms to use the BLIS to simplify and improve the detection of branches created at bifurcations.