Robustness of a Network Formed of Spatially Embedded Networks
  We present analytic and numeric results for percolation in a network formed of interdependent spatially embedded networks. We show results for a treelike and a random regular network of networks each with $(i)$ unconstrained interdependent links and $(ii)$ interdependent links restricted to a maximum length, $r$. Analytic results are given for each network of networks with unconstrained dependency links and compared with simulations. For the case of two spatially embedded networks it was found that only for $r>r_c\approx8$ does the system undergo a first order phase transition. We find that for treelike networks of networks $r_c$ significantly decreases as $n$ increases and rapidly reaches its limiting value, $r=1$. For cases where the dependencies form loops, such as in random regular networks, we show analytically and confirm through simulations, that there is a certain fraction of dependent nodes, $q_{max}$, above which the entire network structure collapses even if a single node is removed. This $q_{max}$ decreases quickly with $m$, the degree of the random regular network of networks. Our results show the extreme sensitivity of coupled spatial networks and emphasize the susceptibility of these networks to sudden collapse. The theory derived here can be used to find the robustness of any network of networks where the profile of percolation of a single network is known. 
Louis M. Shekhtman, Yehiel Berezin, Michael M. Danziger, Shlomo Havlin
physics.soc-ph