Timothy excercise

Question

Comments

See http://gatecse.in/closure-property-of-language-families/

'a' and 'b' seems correct. But then what's the answer?

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i think its b

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Options a and b both are correct because say we have languages A and B, then using union and complement property we can prove that its closed under intersection too. So option A is correct. For the same reason B is also correct but not C.
proof:-
let A,B belongs to Language L

then A', B' and (A + B) too belong to L. (since closed under union and complementation)

=> (A'+B')' belongs to L
=> (A intersection B) belongs to L.

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If the class of languages are like regular,CSL,REC , then option D is true,

else it may or may not be true.