Unboundedness and Downward Closures of Higher-Order Pushdown Automata

13 Jul 2015  ·  Matthew Hague, Jonathan Kochems, C. -H. Luke Ong ·

We show the diagonal problem for higher-order pushdown automata (HOPDA), and hence the simultaneous unboundedness problem, is decidable. From recent work by Zetzsche this means that we can construct the downward closure of the set of words accepted by a given HOPDA. This also means we can construct the downward closure of the Parikh image of a HOPDA. Both of these consequences play an important role in verifying concurrent higher-order programs expressed as HOPDA or safe higher-order recursion schemes.

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Formal Languages and Automata Theory F.4.3

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