Peter Linz Edition 4 Exercise 8.1 Question 8 (Page No. 212)

Question

Determine whether or not the following languages are context-free.
(a) $L=$ {$a^nww^Ra^n : n ≥ 0, w ∈$ {$a,b$}*}
(b) $L=$ {$a^nb^ja^nb^j : n ≥ 0, j ≥ 0$}.
(C) $L=$ {$a^nb^ja^jb^n : n ≥ 0, j ≥ 0$}.
(d) $L=$ {$a^nb^ja^kb^l : n + j ≤ k + l$}.
(e)$L=$ {$a^nb^ja^kb^l : n ≤ k, j ≤ l$}.
(f) $ L=$ {$a^nb^nc^j : n ≤j$}.
(g) $L=$ {$w ∈$ {$a, b, c$}* $: n_a(w)= n_b (w)=2n_c(w)$}.

Comments

b,e,f,g  are not context free language

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What about a

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 'a' is a CFL. A CFG can be generated for it. 

Refer to: http://www.cs.williams.edu/~tom/courses/361/notes/Lect17.pdf    page: 5

for more clarification.

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why f is not cfg???