Feasibility of Nash-Moser iteration for Cheng-Yau-type gradient estimates of nonlinear equations on complete Riemannian manifolds

Abstract: In this manuscript, we employ the Nash-Moser iteration technique to determine a condition under which the positive solution $u$ of the generalized nonlinear Poisson equation $$\operatorname{div} (\varphi(|\nabla u|^2)\nabla u) + \psi(u^2)u = 0,$$ on a complete Riemannian manifold with Ricci curvature bounded from below can be shown to satisfy a Cheng-Yau-type gradient estimate. We define a class of $\varphi$-Laplacian operators by $\Delta_{\varphi}(u):=\operatorname{div} (\varphi(|\nabla u|^2)\nabla u)$, where $\varphi$ is a $C^2$ function under some certain growth conditions. This can be regarded as a natural generalization of the $p$-Laplacian, the $(p,q)$-Laplacian and the exponential Laplacian, as well as having a close connection to the prescribed mean curvature problem. We illustrate the feasibility of applying the Nash-Moser iteration for such Poisson equation to get the Cheng-Yau-type gradient estimates in different cases with various $\varphi$ and $\psi$. Utilizing these estimates, we proves the related Harnack inequalities and a series of Liouville theorems. Our results can cover a wide range of quasilinear Laplace operator (e.g. $p$-Laplacian for $\varphi(t)=t^{p/2-1}$), and Lichnerowicz-type nonlinear equations (i.e. $\psi(t) = At^{p} + Bt^{q} + Ct\log t + D$).