TOC doubts test series

Question

State whether statement is true or false.

1. (a+b)*.aⁿbⁿ is regular .
2. Language produced by grammar S -> AaB A->aC | $\epsilon$  B->aB|bB|$\epsilon$  C->aCb|$\epsilon$ is regular.
3. Language produced by grammar S->AB  A->aA|$\epsilon$  B->aBb|$\epsilon$ is regular.
4. aⁿbⁿcaⁿbⁿ n>=0 is CFL.
5. L1 = {w | the number of occurrences of "ab" in w is equal to number of occurrences of "ba" }
6. Given a grammar, checking if the grammar is not regular is decidable problem.

Comments

For 6 checking for a grammar is regular is undecidable but checking if a grammar is not regular is decidable ?Because regularity of CFL,CSL,RE,REC is undecidable .

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we are talking about grammer not language , we have rule for regular grammer ryt?

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Ok thanks got it.