For this system, the overall transfer function is given by
Thus the closed-loop poles (roots of the characteristic equation) of the transfer function are the solutions to the equation 1 + G(s)H(s) = 0. The principal feature of this equation is that roots may be found wherever G(s)H(s) = -1.
In systems without pure delay, the product G(s)H(s) = -1 is a rational polynomial function and may be expressed as