Remarks on non-compact complete Ricci expanding solitons

In this paper, we study gradient Ricci expanding solitons $(X,g)$ satisfying $$Rc=cg+D^2f,$$ where $Rc$ is the Ricci curvature, $c<0$ is a constant, and $D^2f$ is the Hessian of the potential function $f$ on $X$. We show that for a gradient expanding soliton $(X,g)$ with non-negative Ricci curvature, the scalar curvature $R$ has at least one maximum point on $X$, which is the only minimum point of the potential function $f$. Furthermore, $R>0$ on $X$ unless $(X,g)$ is Ricci flat. We also show that there is exponentially decay for scalar curvature for $\epsilon$-pinched complete non-compact expanding solitons.