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Given the alphabet $Λ = \{a, b\}$ and this non-regular language:

$$L=\{aba^nb^ma^n|n,m>0\}$$

what's the best way to generate the grammar? Every string has to start with $ab$, then we have $aba$ and this part has to have the same number of $a$. In words that is clear, but I'm stuck; should I first consider the fixed part $ab$, or is better to produce the $a^nb^ma^n$ part and then add the $ab$? I need a method basically.

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You should think of this language as all words of the form $$ xa^nya^n, $$ where $x=ab$ and $y \in b^*$. You can now modify a grammar for $a^nb^n$ to generate your language.

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