A proof of Ehrenfeucht's conjecture
Theoretical Computer Science
The unsolvability of the Equivalence Problem for Λ-Free nondeterministic generalized machines
Journal of the ACM (JACM)
Handbook of Formal Languages
The Power of Commuting with Finite Sets of Words
Theory of Computing Systems
Undecidability in Integer Weighted Finite Automata
Fundamenta Informaticae
Unique Decipherability in the Monoid of Languages: An Application of Rational Relations
CSR '09 Proceedings of the Fourth International Computer Science Symposium in Russia on Computer Science - Theory and Applications
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We show that it is undecidable whether two finite substitutions agree on the binary language a*b. This in particular means that equivalence of nondeterministic finite transducers is undecidable even for two-state transducers with unary input alphabet and whose all transitions start from the initial state.