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# 13 - 13 Closure Properties of CFLs Theorem 1 CFLs are...

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13: Closure Properties of CFLs Theorem 1 CFLs are closed under 1. union, 2. concatenation, 3. Kleene Star. Proof Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ), and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be the two CFG generating the CFLs L 1 and L 2 . Rename the nonterminals, if necessary, so that V 1 Σ 1 and V 2 Σ 2 are disjoint sets. 1. Let G = ( V 1 V 2 ∪ { S } , Σ 1 Σ 2 , R 1 R 2 ∪ { S S 1 , S S 2 } , S ). Prove that L ( G ) = L ( G 1 ) L ( G 2 ): w L ( G ) iff S G w . iff S G S 1 G w or S G S 2 G w . iff S 1 G w or S 2 G w . iff S 1 G 1 w or S 2 G 2 w , since S V 1 V 2 and ( V 1 Σ 1 ) ( V 2 Σ 2 ) = . iff w L ( G 1 ) or w L ( G 2 ). iff w L ( G 1 ) L ( G 2 ). 1

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2. Let G = ( V 1 V 2 ∪ { S } , Σ 1 Σ 2 , R 1 R 2 ∪ { S S 1 S 2 } , S ). Prove that L ( G ) = L ( G 1 ) L ( G 2 ). 3. Let G = ( V 1 ∪ { S } , Σ 1 , R 1 ∪ { S e, S SS 1 } , S ). Prove that L ( G ) = L ( G 1 ) . Note: CFLs are not closed under intersection and comple- mentation. (see next set of notes).
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