cfg pda toc

Question

how $a^nb^nc^n$ n>=1 is not CFL....??

Comments

A CFL is accepted by a stack, means using push and pop operations only, the every string of language should be accepted. Can you accept the above language using stack only?

For string $aaabbb$, you can use a stack saying Push all a's, then Pop() for every b in input string. and if we find our stack empty after seeing all b's, we can say Yeah!! string got accepted.

Can you do something similar to this for $aaabbbccc$?

Answer 1

we can not compare equal no. of  a ,b and c in stack 

we can only compare two equal no. of symbol i.e. aⁿbⁿ

Comments

cant we leave 'c' uncompared.....

there is a question L={$a^{n}b^{n}c^{m}$, n,m>=1}

this language is a context free as we leave 'c' uncompared here...?

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@Anmol Verma  language in question i.e L₁={aⁿbⁿcⁿ,n>=1} is different from

L₂₌{aⁿ,bⁿ,c^m,n,m>=1}.

in L₁, we need 2 stacks because we have equal no. of a,b & c While in L_2,we don't need equal no of a,b,c (though equal no of a & b but not c). Here, we can leave c 'Uncomapred'.

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isn't $L_{1}$ a subset of $L_{2}$...???

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Subset of a context free language may or may not be context free.

a^n n is a natural number is context free

a^n where n is a prime number is not.

Though latter is the subset of the former. :-)