If a self oscillating electrical (passive) system is excited at two nodes A and B by a sinusoidal current, and if this system has one degree of freedom, then the response of the system is maximal at the resonance frequency. Quantitatively, this means that the ratio of the exciting complex voltage by the exciting complex current is maximal at the resonance frequency. In other words, 1/|Z(w)| is minimal at the resonance frequency w, with Z the complex impedance of the system seen as a circuit A-B (I believe this is common knowledge but I have no source). For example, the impedance of an LC tank circuit is Z = j w L/(1 - w2LC), hence 1/|Z| is minimal whenever w2LC = 1. I wonder if there is not a more general statement for electrical systems with several degree of freedom. Something like: the frequency of the vibrational modes are the local minimum of the function 1/|Z(w)|, or what is the same, of 1/|Z(w)|2. Any insight, references?