On Hopf bifurcations in singularly perturbed systems

It has been shown recently that, under some generic assumptions, there exists a Hopf curve (lambda) = (lambda) (( epsilon)) for singularly perturbed systems of the form x. = f (x, y, (lambda)), (epsilon)/y . = g(x, y, (lambda)) near the singular surface defined by det g/sub v/ = 0. In this note, we are concerned with the Hopf curve and obtain three results: 1) we prove that the eigenvalue crossing condition for the Hopf curve holds without additional assumption; 2) we provide an improved form of an existing derivative formula for the Hopf curve which is more suitable for practical computations; and 3) we give a quite precise description of the spectrum structure of the linearization along the Hopf curve. All three results (stated in the main theorem) are useful for a better understanding of Hopf bifurcations in singularly perturbed systems. Our analysis is based on a factorization of parameter dependent polynomials (Lemma 2.3).