Pumping lemma (Regular language)

浪尽此生 提交于 2019-12-05 09:08:56

The main idea of the pumping lemma is to tell you that when you have a regular language L with infinite number of terms there is a pattern in the language that repeats forever.

The regular expression associated with that language will contain KLEENE-STAR(pattern).

The automaton associated with that regular expression (and language) will contain a loop.

The proof is done using the pigeon principle.

This

is very suggestive.

Note that all terms must start in q0 and end in qn in this case. So, the automata defining the language is finite (max N states), so there are a limited number of states, but the words (i.e. terms) can have >N letters. The pigeon principle tells us that there must be a state that is reached 2 times, so at that state a loop will be present.

In your notation, you can make the correspondence with the image so:

  • your u is x from image

  • v is y in image

  • w is z from image

To arrive from q0 to qn, you can use any of the strings from the set: { uw , uvw, uvvw, uvvvw, ... }.

In this particular case the pattern P is y, the set X is {xz xyz xyyz xyyyz ...} and S is length(x)+length(y).

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