About: Negative imaginary systems

Negative imaginary (NI) systems theory was introduced by Lanzon and Petersen in. A generalization of the theory was presented in In the single-input single-output (SISO) case, such systems are defined by considering the properties of the imaginary part of the frequency response G(jω) and require the system to have no poles in the right half plane and > 0 for all ω in (0, ∞). This means that a system is Negative imaginary if it is both stable and a nyquist plot will have a phase lag between [-π 0] for all ω > 0.