Cumulants and Limit Theorems for $q$-step walks on random graphs of long-range percolation radius model

We study cumulants of numbers of $q$-step walks on Erdös-Rényi-type random graphs of long-range percolation radius model in the limit when the number of vertices $N$, concentration $c$, and the interaction radius $R$ tend to infinity. These cumulants can be associated with a formal cumulant expansion of the free energy of matrix models of exponential random graphs widely known in mathematical and theoretical physics. We show that in three different asymptotic regimes, the limiting values of $k$-th cumulants ${\cal F}_k^{(q)}$ exist and can be associated with one or another family of tree-type diagrams, in dependence of the asymptotic behavior of parameters $cR/N$ for $q$-step non-closed walks and $c^2R/N^2$ for 3-step closed walks, respectively. In certain cases, we obtain ${\cal F}_k^{(q)}$ in explicit form. These results allow us to prove Limit Theorems for the number of non-closed walks and for the number of triangles in corresponding ensembles of large random graphs. As a consequence, we indicate an asymptotic regime when in random graphs that we consider, the average vertex degree remains bounded while the total number of triangles infinitely increases, thus rigorously solving a graph collapse problem known in applications.