Zhong Li 0003 Fengde Chen Extinction in two dimensional nonautonomous Lotka-Volterra systems with the effect of toxic substances. 684-690 2006 182 Appl. Math. Comput. 1 https://doi.org/10.1016/j.amc.2006.04.034 db/journals/amc/amc182.html#LiC06