GATE | GATE-CS-2017 (Set 1) | Question 52

Consider the following languages over the alphabet ∑= {a,b,c}. Let L₁ = {aⁿ bⁿc^m | m, n >= 0 } and L₂ = {a^mbⁿcⁿ| m, n >= 0}.

Which of the following are context-free languages ?

I. L₁ ∪ L₂
II. L₁ ∩ L₂

(A) I only
(B) II only
(C) I and II
(D) Neither I nor II


Answer: (A)

Explanation: Union of context free language is also context free language.

L1 = { aⁿ bⁿ c^m| m >= 0 and n >= 0 } and
L2 = { a^m bⁿcⁿ| n >= 0 and m >= 0 }

L3 = L1 ∪ L2 = { aⁿbⁿc^m∪ a^mbⁿcⁿ| n >= 0, m >= 0 } is also context free.

L1 says number of a’s should be equal to number of b’s and L2 says number of b’s should be equal to number of c’s. Their union says either of two conditions to be true. So it is also context free language.

Intersection of CFG may or may not be CFG.
L3 = L1 ∩ L2 = { aⁿbⁿcⁿ| n >= 0 } need not be context free.



This solution is contributed by parul sharma.

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