  Let $(V,\mu)$ be an infinite, connected, locally finite weighted graph. We
study the problem of existence or non-existence of positive solutions to a
semi-linear elliptic inequality \begin{equation*} \Delta u+u^{\sigma}\leq0\quad
\text{in}\,\,V, \end{equation*} where $\Delta$ is the standard graph Laplacian
on $V$ and $\sigma>0$. For $\sigma\in(0,1]$, the inequality admits no
nontrivial positive solution. For $\sigma>1$, assuming condition
\textbf{($p_0$)} on $(V,\mu)$, we obtain a sharp condition for nonexistence of
positive solutions in terms of the volume growth of the graph, that is
\begin{equation*} \mu(o,n)\lesssim n^{\frac{2\sigma}{\sigma-1}}(\ln
n)^{\frac{1}{\sigma-1}}
  \end{equation*}
  for some $o\in V$ and all large enough $n$. For any $\varepsilon>0$, we can
construct an example on a homogeneous tree $\mathbb T_N$ with $\mu(o,n)\approx
n^{\frac{2\sigma}{\sigma-1}}(\ln n)^{\frac{1}{\sigma-1}+\varepsilon}$, and a
solution to the inequality on $(\mathbb T_N,\mu)$ to illustrate the sharpness
  of $\frac{2\sigma}{\sigma-1}$ and $\frac{1}{\sigma-1}$.

Analysis of PDEs