Improving network robustness by edge modification

An important property of networked systems is their robustness against removal of network nodes, through either random node failure or targeted attack. Although design methods have been proposed for creating, ab initio, a network that has optimal robustness according to a given measure, one is often instead faced with an existing network that cannot feasibly be substantially modified or redesigned, yet whose robustness can be improved by a lesser degree of modification. We present empirical results that show how robustness, as measured either by the size of the largest connected component or by the shortest path length between pairs of nodes, is affected by several different strategies that alter the network by rewiring a fraction of the edges or by adding new edges. We find that a modest alteration of an initially 'scale-free' network can usefully improve robustness against attack, particularly when the fraction of attacked nodes is small, and we identify modification schemes that are most effective for this purpose.