Real axis roots are determined from the phase
relationship in Eq. (16). Assume &,(@ 2 0. A real
axis factor s+zi(Q, or s+pi, contributes a phase angle of
180 deg only when s lies to the left of the root. An odd
number of these contributions is necessary to satisfy
Eq. (16). Recall the roots -zi(Q are not stationary, but
moving and can thus pass by stationary open-loop pole
locations. Real axis roots "trapped" between such
points must undergo a reversal of direction to preserve
the phase requirement. Therefore, Rule 3 states "Real
axis roots lie to the left of an odd number of roots -zi(Q
and -pi". Additional rules covering departure-arrival
angles, asymptotes, imaginary axis crossings, etc. can
be found in Ref. 8.