Iranian Mathematical Society (IMS) Bulletin of the Iranian Mathematical Society 1017-060X 37 No. 3 2011 09 15 Permanence and global asymptotic stability of a delayed predator-prey model with Hassell-Varley type functional response197 215 en K. Wang Y. L. Zhu 2009 07 21 Permanence asymptotic stability comparison theorem Lyapunov functional