Inclusion between the frontier language of a non-deterministic recursive program scheme and the Dyck language is undecidable.

In 1970's, Nivat studied recursive program schemes (a.k.a order-1 higher-order recursion schemes in modern terminology), first-order tree grammars for generating possibly infinite trees. We consider the inclusion problem between the frontier language of a non-deterministic recursive program scheme (equivalently, an order-2 word language or indexed language) and the Dyck language, and prove that it is undecidable by a reduction from the undecidability of Hilbert's 10th problem. Essentially the same result has recently been proved by Uezato and Minamide, but our proof is arguably more direct, demonstrating the expressive power of higher-order grammars. [ABSTRACT FROM AUTHOR]