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(from 'An Introduction to Formal Languages and Automata' by Peter Linz)

What I do not understand, is why we have done our best to make sure that the condition (8.2) holds. Why is this restriction important/useful?


1 Answer 1


The restriction is definitely useful. It provides you with an additional guarantee on the structure of the decomposition of $w$, which you can exploit in your proofs.

For example the language $L = \{a^n b^k c^n d^k \mid n, k \ge 0\}$ is not context-free and a possible proof using the pumping lemma is as follows: suppose that $L$ is context free, and let $m$ be its pumping length. Consider the word $w=a^m b^m a^m b^m$. The pumping lemma ensures that there is a decomposition of $w = uvxyz$ with $|vxy| \le m$, and this implies that $vxy$ cannot contain both a $a$ and a $c$, nor both a $b$ and a $d$. As a consequence $uv^2xy^2z \not\in L$, which provides the sought contradiction.

Notice that the above proof would not work without using $|vxy| \le m$, since we could pick $u=a^{m-1}$, $v=a$, $x=b^m$, $y=c$, and $z=c^{m-1}d^k$, which satisfies $uv^i x y^i z \in L$ for all $i \in \mathbb{N}$.


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