Gradient Estimates for the Nonlinear Parabolic Equation with Two Exponents on Riemannian Manifolds

In this paper, we study the nonlinear parabolic equation with two exponents on complete noncompact Riemannian maniflods. The special types of such equation include the Fisher-KPP equation, the parabolic Allen-Cahn equation and the Newell-Whitehead equation. We get the Souplet-Zhang's gradient estimates for the positive solutions to such equation. We also obtain the Liouville theorem for positive ancient solutions. Our results extend those of Souplet-Zhang (Bull. London. Math. Soc. 38 (2006), 1045-1053) and Zhu (Acta Math. Sci. Ser. B 36 (2016), no. 2, 514-526).