The effect of positive (that is, real open-right-half-plane)
zeros is evident in the step response of a system. In particular,
Figure 2 shows a step response that departs in the
nonasymptotic direction; this phenomenon, which is
equivalent to initial error growth, is called initial
undershoot. Note that initial undershoot is defined only for
a step response whose initial and asymptotic values are
different. A classical result, proved in [7]–[9], states that
the step response of an asymptotically stable, strictly proper
transfer function exhibits initial undershoot if and only
if the system has an odd number of positive zeros