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We consider the simultaneous propagation of two contagions over a social
network. We assume a threshold model for the propagation of the two contagions and
use the formal framework of discrete dynamical systems. In particular, we study an
optimization problem where the goal is to minimize the total number of infected nodes
subject to a budget constraint on the total number of nodes that can be vaccinated.
While this problem has been considered in the literature for a single contagion, our
work considers the simultaneous propagation of two contagions. Since the optimization
problem is NP-hard, we develop a heuristic based on a generalization of the set cover
problem. Using experiments on three real-world networks, we compare the performance
of the heuristic with some baseline methods.

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