A note on the nonexistence of positive supersolutions to elliptic equations with gradient terms

We prove that if the elliptic problem $-\Delta u+b(x)|\nabla u|=c(x)u$ with $c\ge0$ has a positive supersolution in a domain $\Omega$ of $\IR^{N\ge 3}$, then $c,b$ must satisfy the inequality $$\sqrt{ \int_\Omega c\phi^2}\le \sqrt{ \int_\Omega | \nabla\phi|^2}+\sqrt{ \int_\Omega \frac{b^2}{4}\phi^2},~~~\phi \in C_c^\infty(\Omega).$$ As an application, we obtain Liouville type theorems for positive supersolutions in exterior domains when $c(x)-\frac{b^2(x)}{4}>0$ for large $|x|$, but unlike the known results we allow the case $\liminf_{|x|\rightarrow\infty}c(x)-\frac{b^2(x)}{4}=0$. Also the weights $b$ and $c$ are allowed to be unbounded. In particular, among other things, we show that if $\tau:=\limsup_{|x| \rightarrow\infty}|xb(x)|<\infty$ then this problem does not admit any positive supersolution if $$\liminf_{|x| \rightarrow\infty}|x|^2c(x)> \frac{(N-2+\tau)^2}{4},$$ and, when $\tau=\infty,$ we have the same if $$\limsup_{R\rightarrow\infty} R\Big(\frac{ \inf_{R<|x|<2 R} (c(x)-\frac{b(x)^2}{4})}{\sup_{\frac{R}{2}<|x|<4 R}|b(x)|}\Big)=\infty.$$