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I want to prove that the language $L = \{ a^{n}(ab)^{{n}^{2}}b^{n} \mid n \geq 0 \}$ is not context-free by using Parikh's theorem.

My first assumption is that the $(ab)^{{n}^{2}}$ part cannot be written as a subset of linear vectors. From this it follows that $\psi(L)$ is not semi-linear.

My problem is to show that by a formal proof.

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If $L$ were context-free then, applying the homomorphism mapping both letters to $a$, also the following language would be context-free: $$ L' = \{ a^{2n^2+2n} : n \geq 0 \}. $$ According to Parikh's theorem, a unary language is context-free iff the corresponding subset of $\mathbb{N}$ is eventually periodic. Yet the sequence $2n^2+2n$ has arbitrarily long gaps, since $2(n+1)^2+2(n+1)-2n^2-2n = 4n+4$. Therefore the set $\{2n^2+2n : n \geq 0\}$ is not eventually periodic, and so $L'$ is not regular or context-free.

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  • $\begingroup$ Thank you for the answer! Can you tell me (or give me reference for) what is means that a corresponding subset must be periodic and why it must fulfill that condition for context-free languages? $\endgroup$ – xvzwx Jan 14 '17 at 17:09
  • $\begingroup$ A subset $S$ of $\mathbb{N}$ is eventually periodic if there exist $n_0,p \geq 1$ such that for all $n \geq n_0$, we have $n \in S$ iff $n+p \in S$. The subset corresponding to a unary language has to be eventually periodic due to Parikh's theorem. $\endgroup$ – Yuval Filmus Jan 14 '17 at 17:38

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