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Showing balanced parens to be irregular

September 26, 2013

This is for young people doing CS and are wondering how to use the Pumping Lemma to verify if a formal language is regular or not :-). So let us have a theorem about the balanced parenthesis language :
L_{()} = \{ (^n \varphi )^n | n \geq 1 \wedge \varphi \neq \Lambda \} is not a regular language.
Some text books use \Lambda to denote the empty string so here \varphi is none empty. The method of proof is by contraction (Reduction ad Absurdium – RAA). So we assume the language is regular and hence can be pumped but show it to be otherwise, or not true by arriving at a contradiction. By the Pumping Lemma (PLem), \exists m so that m can be used to split w \in L_{()} into parts. We actually use this m to form w. We can do this because the Lemma says this m is valid for any w, |w| \geq m.
Assume L_{()} to be regular. Then by PLem,  there exists an m such that for w \in L_{()}, |w| \geq m. Consider now the w formed by setting n = m. Then we have w = (^m \varphi )^m. Further we know that |w| = 2m+1 > m satisfying PLem premise, so it can be applied. As per PLem,  we can break w into x, y, z components. As per PLem also |xy| \leq m. Looking at the form of our w this implies that xy must be composed of all left parenthesis, i.e  xy = (^m, then y=(^p for some p < m.  As per PLem, we can pump y  for any k \geq 0.  So let k =0 then the new w = (^{m-p} \varphi )^m. But this implies that |(^{m - p}| = m - p \neq m =|)^m|, i.e. parentheses are not balanced. But this means w \not \in L_{()}. Contradiction. At this point we have found a k where the decomposition results in the string outside of L_{()}and we can stop.

Anyway consider now also y pumped upwards to k. Then we have the new w = (^{m-p}(^{pk} \varphi )^m. Looking at the left of \varphi we have |(^{m-p}(^{pk}|= m - p + pk \neq m= |)^m|. Again the parentheses do not balance out. Thus w \not \in L_{()}. Contradiction.


Feedback is appreciated – let me know if it has helped/not helped. Thanks.

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