In 1] the theory of secondary instability for plane Poiseuille flow was proposed. In this paper the weakly nonlinear theory
is used to investigate the same problem. The results are consistent with those of the theory of secondary instability. It
indicates that for plane Poiseuille flow there exists a certain intrinsic relation between the weakly nonlinear theory and
the theory of secondary instability.
This work is supported by the Science Fund of the Chinese Academy of Sciences.