According to the Wikipedia page on finite-state transducers, it is undecidable whether two finite-state transducers are equivalent. I find this result striking, since it is decidable whether two finite-state automata are equivalent to one another.
Unfortunately, Wikipedia doesn't provide any citations that provide a justification for this result. Does anyone know of a proof of this result? Alternatively, if the article is incorrect, does someone know an algorithm that could be used to show equivalence?