where ⑀ = ͉͑V , W , ͉͒ so that ⑀ provides a count of the number of V, W, and factors in any one term. Substitution of Eq. ͑17͒ into Eqs. ͑8b͒ and ͑8c͒ leads to the following reduced system of evolution equations, which describe the dynamics on the center manifold