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Just to make it clear. (since my book doesn't mention anything like this)

Suppose we have a context free grammar $G=(V,T,P,S)$. where $T=\{a,b\}$ (The other sets doesn't really matter).

Since $\Rightarrow^*$ is the reflexive transitive closure of the relation $\Rightarrow$ on the set $(V\cup T)^*$, I see no problem in writing a thing like $ab \Rightarrow^* ab$ (where $ab\in T^*$ and there are no variables on the left hand side of the arrow) since the $\Rightarrow^*$ relation is reflexive.

Now I want to ask wether it common to write a thing like $ab \Rightarrow^* ab$?

By definition I see no problem.

Thanks.

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There is absolutely no problem writing $ab \Rightarrow^* ab$. It definitely happens in proofs.

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  • $\begingroup$ Thanks a lot, Indeed I have encountered this in my proof and it is a lot easier to use this notation especially if there are several cases to consider (for instance $X$ may be a terminal word or a variable that derives a terminal word). $\endgroup$ – MathNerd Jan 4 '16 at 18:57

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