  In this paper, let $G$ be a Cayley graph of a discrete group of polynomial
growth with homogeneous dimension $N\geq3$. We study the Choquard type equation
on $G$: \begin{equation} \Delta u+(R_{\alpha}\ast\mid u\mid^{p})\mid
u\mid^{p-2}u=0, \end{equation} where $\alpha\in(0,N)$, $p>\frac{N+\alpha}{N-2}$
and $R_{\alpha}$ stands for the Green's function of the discrete fractional
Laplace operator, which has same asymptotics as the Riesz potential. We prove
the discrete Hardy-Littlewood-Sobolev inequality on such Cayley graphs, and by
the discrete Concentration-Compactness principle we prove the existence of
extremal functions for the corresponding Sobolev type inequalities in
supercritical cases, which yields a positive ground state solution of the above
Choquard type equation. Moreover, we obtain positive ground state solutions of
Choquard type equations with $p$-Laplace, biharmonic and $p$-biharmonic
operators etc.
