Physical Reality of Complex Numbers is Proved by Research of Resonance

On the example of research of the simplest electric LCR-circuits it is shown that they have different frequencies corresponding to various definitions of resonance at real frequencies. Consequently, the resonance theory is imperfect. Proceeding from the geometric sense of Cassinian oval, the resonance theory based on complex frequencies free of the aforementioned contradictions is suggested. The physical meaning of resonance on complex frequencies is explained. The physical reality of resonance on complex frequencies is concluded.