by Yeo et al. in the potential  ow over a single-layer viscoelastic wall. On the other hand, no cusp points have been found at the ®i contours for an undamped plate (dD 0). All of the ®i 6D 0 contours are discontinuousin upper-half plane; ! roots jump to the opposite side of the real axis before the contours form cusps. It, therefore, indicates that a potential  ow over an inŽ nitely long undamped bending plate does not admit absolute instability. A similar situation can also be seen for a membrane (T 6D 0, DD 0). A potential  ow over an undampedmembrane admits static temporal instability,whereas a  ow over dampedmembrane admits only an absolute instabilitymode.