A Direct Proof of the Inherent Ambiguity of a Simple Context-Free Language
Herman A. Maurer · Journal of the ACM · 1969
A direct and self-contained proof is given of the inherent ambiguity of the context-free language L = { a i b i c i ∣ i,j > 1} ∪ { a i b i c i ∣ i,j > 1}, which is the solution to an open problem pointed out by Ginsburg.