Determining whether a context-free grammar is ambiguous can easily be shown to be undecidable using a reduction from the most famous problem named for this man.
This man formulated a Turing-complete triplet with initial words and production rules that generates a formal language, and the priority method of proof was developed when solving a question asked by him. Another problem posed by this man gives sigma with at least two symbols and two sequences of strings over sigma and asks whether there is some sequence of indices such that the two sequences of strings are equivalent over those indices. This namesake of a (*) "canonical system" also asked whether there exists a recursively enumerable set with Turing degree less than the degree of the halting problem. Determining whether a context-free grammar is ambiguous can easily be shown to be undecidable using a reduction from the most famous problem named for this man. For 10 points, name this computer scientist who posed an undecidable correspondence problem.
This man formulated a Turing-complete triplet with initial words and production rules that generates a formal language, and the priority method of proof was developed when solving a question asked by him.
Emil Leon Post