Existence and convergence of solutions for nonlinear biharmonic equations on graphs

In this paper, we first prove some propositions of Sobolev spaces defined on a locally finite graph $G=(V,E)$, which are fundamental when dealing with equations on graphs under the variational framework. Then we consider a nonlinear biharmonic equation $$\Delta^{2} u -\Delta u+(\lambda a+1)u= |u|^{p-2}u$$ on $G=(V,E)$. Under some suitable assumptions, we prove that for any $\lambda>1$ and $p>2$, the equation admits a ground state solution $u_{\lambda}$. Moreover, we prove that as $\lambda\rightarrow +\infty$, the solutions $u_{\lambda}$ converge to a solution of the equation \begin{align*} \begin{cases} \Delta^{2}u -\Delta u+u = |u|^{p-2}u, &\text{in}\ \ \Omega, u=0, &\text{on}\ \ \partial\Omega, \end{cases} \end{align*} where $\Omega=\{x\in V: a(x)=0\}$ is the potential well and $\partial\Omega$ denotes the the boundary of $\Omega$.