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Stably computable predicates are semilinear

We consider the model of population protocols introduced by Angluin et al. [2], in which anonymous finite-state agents stably compute a predicate of their inputs via two-way interactions in the all-pairs family of communication networks. We prove that all predicates stably computable in this model (and certain generalizations of it) are semilinear, answering a central open question about the power of the model.