Bisimulation Equivalence of Pushdown Automata Is Ackermann-Complete

Abstract Deciding bisimulation equivalence of two pushdown automata is one of the most fundamental problems in formal verification. Though Sénizergues established decidability of this problem in 1998, it has taken a long time to understand its complexity: the problem was proven to be non-elementary in 2013, and only recently, Jančar and Schmitz showed that it has an Ackermann upper bound. We improve the lower bound to Ackermann-hard, and thus close the complexity gap.

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