For finite-dimensional maps, an unstable orbit in a neighborhood of an unstable fixed point can be stabilized by adjusting parameters so that the orbit goes to the fixed point along the straight line connecting the orbit (at a given time) and the fixed point [Yang Ling, Liu Zengrong and Jian-min Mao, Phys. Rev. Lett. 84, 67 (2000)]. This is called straight-line stabilization. In this paper, we derive the expression for the region of stabilization, i.e., the region within which the straight-line stabilization method is valid. For two-dimensional maps, the parameter adjustments needed by the stabilization method are explicitly given for nine cases. Stabilization of unstable flows, with or without introducing a Poincare map, is also investigated.
For the nonlinear Schrödinger equation, the Korteweg-de Vries equation, and the modified Korteweg-de Vries equation, periodic exact solutions are constructed from their stationary periodic solutions, by means of the Bäcklund transformation. These periodic solutions were not written down explicitly before to our knowledge. Their asymptotic behavior when t-->-infinity is different from that when t-->infinity. Near t=0, the spatial-temporal pattern can change abruptly, and rational solitons can appear randomly in space and time. They correspond to new types of "homoclinic orbits" due to different asymptotic behaviors in time.
