Toroidal normal forms for bifurcations in retarded functional   differential equations I: Multiple Hopf and transcritical/multiple Hopf   interaction
  For finite-dimensional bifurcation problems, it is well-known that it is possible to compute normal forms which possess nice symmetry properties. Oftentimes, these symmetries may allow for a partial decoupling of the normal form into a so-called ``radial'' part and an ``angular'' part. Analysis of the radial part usually gives an enormous amount of valuable information about the bifurcation and its unfoldings. In this paper, we are interested in the case where such bifurcations occur in retarded functional differential equations, and we revisit the realizability and restrictions problem for the class of radial equations} by nonlinear delay-differential equations. Our analysis allows us to recover and considerably generalize recent results by Faria and Magalhaes and by Buono and Belair. 
Younsun Choi and Victor G. LeBlanc