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Ogden’s Lemma for CFL Ogden’s Lemma:

 

Let L be an infinite Context Free Language, then there exists some positive integer n

 

Such that for any string ω ∊ L with |ω| ≥ n, and if we mark at least n positions in ω as distinguished, then we can write ω = uvxyz.

 

Such that :

 

  • x has at least one marked position.

 

  • v and y together contain at least one distinguished position.

 

  • vxy has at most n distinguished position.

 

 

Use of Ogden’s Lemma: 

 

Ogden’s lemma can be used to prove the given languages are not Context Free Language, in that case where pumping lemma for CFL fails. 

 

Example:

 

 

It is not CFL. This proves is not possible for pumping lemma.