Complex extension of potentials and trajectories of poles of the   S-matrix element in the complex momentum plane
  Searching for infrastructure of the quantum mechanical system, we study trajectories of the s-wave poles of the S-matrix element with respect to a real phase $\alpha$ in the complex momentum plane for a complex extension of real potentials by a phase factor $e^{i\alpha}$. This complex extension relates the pole spectrum of the physical system with a potential to the spectrum of another system with the potential of the same shape but of opposite sign. There appear trajectories with the periodicity of $2\pi$, $4\pi$, and $\infty$. The appearance of non-recurrent behavior of the trajectory for the change of phase $\Delta \alpha=2\pi$ is clearly related with the existence of resonance poles for real repulsive potentials. Dynamical changes of trajectory structure are examined. 
M. Kawasaki, T. Maehara and M. Yonezawa