Finsler metrics of weakly isotropic flag curvature

Finsler metrics of scalar flag curvature play an important role to show the complexity and richness of general Finsler metrics. In this paper, on an $n$-dimensional manifold $M$ we study the Finsler metric $F=F(x,y)$ of scalar flag curvature $\mathbf{K}=\mathbf{K}(x,y)$ and discover some equations $\mathbf{K}$ should be satisfied. As an application, we mainly study the metric $F$ of weakly isotropic flag curvature $\mathbf{K}=\frac{3\theta }{F}+\sigma$, where $\theta ={\theta }_i(x)y^i\mathrm{\ne }0$ is a $1$-form and $\sigma =\sigma (x)$ is a scalar function. We prove that in this case, $F$ must be a Randers metric when $dim(M)\ge 3$. Further, without the restriction on the dimension we prove that projectively flat Finsler metrics of such weakly isotropic flag curvature are Randers metrics too.