Algorithmic recognition of infinite cyclic extensions

We prove that one cannot algorithmically decide whether a finitely presented ℤ-extension admits a finitely generated base group, and we use this fact to prove the undecidability of the BNS invariant. Furthermore, we show the equivalence between the isomorphism problem within the subclass of unique ℤ-extensions, and the semi-conjugacy problem for deranged outer automorphisms.

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