Foster's reactance theorem

Foster's reactance theorem [′fȯs·tərz rē′ak·təns ‚thir·əm]

(control systems)

The theorem that the most general driving point impedance or admittance of a network, in which every mesh contains independent inductance and capacitance, is a meromorphic function whose poles and zeros are all simple and occur in conjugate pairs on the imaginary axis, and in which these poles and zeros alternate.