text: We propose an interdependent random geometric graph (RGG) model for interdependent networks. Based on this model, we study the robustness of two interdependent spatially embedded networks where interdependence exists between geographically nearby nodes in the two networks. We study the emergence of the giant mutual component in two interdependent RGGs as node densities increase, and define the percolation threshold as a pair of node densities above which the mutual giant component first appears. In contrast to the case for a single RGG, where the percolation threshold is a unique scalar for a given connection distance, for two interdependent RGGs, multiple pairs of percolation thresholds may exist, given that a smaller node density in one RGG may increase the minimum node density in the other RGG in order for a giant mutual component to exist. We derive analytical upper bounds on the percolation thresholds of two interdependent RGGs by discretization, and obtain 99 confidence intervals for the percolation thresholds by simulation. Based on these results, we derive conditions for the interdependent RGGs to be robust under random failures and geographical attacks.
  @cite_5: Networks of networks are vulnerable: a failure in one sub-network can bring the rest crashing down. Previous simulations have suggested that randomly positioned networks might offer some limited robustness under certain circumstances. Analysis now shows, however, that real-world interdependent networks, where nodes are positioned according to geographical constraints, might not be so resilient.
  @cite_4: We study the cascading failures in a system composed of two interdependent square lattice networks A and B placed on the same Cartesian plane, where each node in network A depends on a node in network B randomly chosen within a certain distance r from the corresponding node in network A and vice versa. Our results suggest that percolation for small r below rmax ! 8 (lattice units) is a second-order transition, and for larger r is a first-order transition. For r < rmax, the critical threshold increases linearly with r from 0.593 at r 1⁄4 0 and reaches a maximum, 0.738 for r 1⁄4 rmax, and then gradually decreases to 0.683 for r 1⁄4 1. Our analytical considerations are in good agreement with simulations. Our study suggests that interdependent infrastructures embedded in Euclidean space become most vulnerable when the distance between interdependent nodes is in the intermediate range, which is much smaller than the size of the system.
  @cite_2: 
summary: In the interdependent networks literature, the model which is closest to ours is the interdependent lattice model, first proposed in @cite_4 and further studied in @cite_5 @cite_2 . In the lattice model, nodes in a network are represented by the open sites (nodes) of a square lattice, where every site is open independently with probability @math . Network links are represented by the bonds (edges) between adjacent open sites. Every node in one lattice is interdependent with one randomly chosen node within distance @math in the other lattice. The distance @math indicates the geographical proximity of the interdependence. The percolation threshold of the interdependent lattice model is characterized as a function of @math , assuming the same @math in both lattices @cite_4 . Percolation of the model where some nodes do not need to have supply nodes was studied in @cite_5 . The analysis relies on quantities estimated by simulation and extrapolation, such as the fraction of nodes in the infinite component of a lattice for any fixed @math , which cannot be computed rigorously. In contrast, we study the percolation of the interdependent RGG model using a mathematically rigorous approach.