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Gradient estimates for positive solutions of the Laplacian with drift
Authors:Benito J Gonzá  lez  Emilio R Negrin
Institution:Departamento de Análisis Matemático, Universidad de La Laguna, 38271 Canary Islands, Spain ; Departamento de Análisis Matemático, Universidad de La Laguna, 38271 Canary Islands, Spain
Abstract:Let $M$ be a complete Riemannian manifold of dimension $n$ without boundary and with Ricci curvature bounded below by $-K,$ where $K\geq 0.$ If $b$ is a vector field such that $\Vert b\Vert \leq \gamma $ and $\nabla b\leq K_{*}$ on $M,$ for some nonnegative constants $\gamma $ and $K_{*},$ then we show that any positive $\mathcal{C}^{\infty }(M)$ solution of the equation $\Delta u(x)+(b(x)|\nabla u(x))=0$ satisfies the estimate

\begin{displaymath}{\frac{{\Vert \nabla u\Vert }^2}{u^2}}\leq \frac{n(K+K_{*})}w+\frac{{\gamma }^2}{w(1-w)}, \end{displaymath}

on $M$, for all $w \in (0,1).$ In particular, for the case when $K=K_{*}=0,$ this estimate is advantageous for small values of $\Vert b\Vert $ and when $b\equiv 0$ it recovers the celebrated Liouville theorem of Yau (Comm. Pure Appl. Math. 28 (1975), 201-228).

Keywords:Gradient estimate  Laplacian with drift  Bochner-Lichn\`erowicz-Weitzenb\"ock formula  Liouville theorem
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