“…A large amount of work related to continuous-time free boundary problems based on the Fisher-KPP equation has emerged. For example, extensions to equations with general nonlinear terms [10,17], time delay [4,33,26], and nonlocal diffusion [3,21], extensions to systems of population or epidemic models [5,8,11,27,30], and see more in a survey paper [6], with great progress has been made in relevant topics.…”