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# Formal languages and automata theory - PowerPoint PPT Presentation

Formal languages and automata theory. Li Fan. Pumping Lemma.

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Let L be a regular set. Then there is a constant n such that if z is any word in L, and |z|>=n, we may write z=uvw in such a way that |uv|<=n, |v|>=1, and for all i>=0, u(v^i)w is in L. Furthermore, n is no greater than the number of states of the smallest FA accepting L.

The set L={0^(i^2) | i is an integer, i>=1}, which consists of all strings of 0’s whose length is a perfect square, is not regular.

Prove that L = {(a^n)(b^k): n > k and n>=0} is not regular.

{0^(2n) | n>=1}

{(0^m)(1^n)(0^(m+n)) | m>=1 and n>=1}

{0^n | n is a prime}

The set of all strings that do not have three consecutive 0’s

{x | x in (0+1)*, and x=x^R} x^R is x written backward; for example, (011)^R=110.

{xwx^R | x,w in (0+1)+}.