The frequency response of a linear system $$G\left( {j\omega } \right)$$ is provided in the tubular form below.

The gain margin and phase margin of the system are

The polar plot of an open loop stable system is shown below. The closed loop systems is

A system with zero initial conditions has the closed loop transfer function $$T\left( s \right) = {{{s^2} + 4} \over {\left( {s + 1} \right)\left( {s + 4} \right)}}.$$ The system output is zero at the frequency

The gain margin of a unity feedback control system with the open loop transfer function $$G\left( s \right) = {{\left( {s + 1} \right)} \over {{s^2}}}$$ is