An Omori-Yau maximum principle for semi-elliptic operators and Liouville-type theorems

We generalize the Omori-Yau almost maximum principle of the Laplace-Beltrami operator on a complete Riemannian manifold $M$ to a second-order linear semi-elliptic operator $L$ with bounded coefficients and no zeroth order term. Using this result, we prove some Liouville-type theorems for a real-valued $C^{2}$ function $f$ on $M$ satisfying $L f \geq F(f)+ H(|\nabla f|)$ for real-valued continuous functions $F$ and $H$ on $\Bbb R$ such that $H(0)=0$.