Summary: This note shows how to check the crossing on the imaginary axis by the eigenvalues of the linearized system of differential equations depending on a real parameter \(\mu\) via feedback system theory. We present simple formulas for both, static (one eigenvalue zero) and dynamic or Hopf (a single pure imaginary pair) bifurcations. Moreover, we show some singular conditions (degeneracies) by continuing the bifurcation curves in the steady-state manifold.