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Turing Machines and Undecidability

GATE CSE 2016 Set 2 · Q18

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Consider the following types of languages: L1L_1 : Regular, L2L_2 : Context-free, L3L_3 : Recursive, L4L_4 : Recursively enumerable. Which of the following is/are TRUE? I. L3‾∪L4\overline{L_3} \cup L_4 is recursively enumerable II. L2‾∪L3\overline{L_2} \cup L_3 is recursive III. L1∗∩L2L_1^* \cap L_2 is context-free IV. L1∪L2‾L_1 \cup \overline{L_2} is context-free

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Solution

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Official answer: D) I, II and III only

Turing Machines and Undecidability

Let us analyze each statement based on closure properties:

  1. Statement I: L3L_3 is a recursive language. Recursive languages are closed under complementation, so L3‾\overline{L_3} is recursive. Since every recursive language is also recursively enumerable (RE), L3‾\overline{L_3} is RE. Recursively enumerable languages are closed under union, so L3‾∪L4\overline{L_3} \cup L_4 is recursively enumerable. Hence, Statement I is TRUE.

  2. Statement II: L2L_2 is a context-free language (CFL). Since every CFL is recursive, L2L_2 is recursive. As recursive languages are closed under complementation, L2‾\overline{L_2} is recursive. Since recursive languages are closed under union, L2‾∪L3\overline{L_2} \cup L_3 is recursive. Hence, Statement II is TRUE.

  3. Statement III: L1L_1 is regular, so by Kleene closure, L1∗L_1^* is also regular. The intersection of a context-free language and a regular language is always context-free. Therefore, L1∗∩L2L_1^* \cap L_2 is context-free. Hence, Statement III is TRUE.

  4. Statement IV: Context-free languages are not closed under complementation, so L2‾\overline{L_2} may not be context-free (it is recursive, but not necessarily context-free). The union of a regular language and L2‾\overline{L_2} is not necessarily context-free. Hence, Statement IV is FALSE.

Therefore, statements I, II, and III are TRUE, which corresponds to option (D).

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This Turing Machines and Undecidability previous year question is Q18 of the GATE CSE 2016 Set 2 paper. Find more GATE CSE PYQs on Turing Machines and Undecidability in Theory of Computation, or browse all GATE CSE PYQs.

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Turing Machines and Undecidability in GATE CSE

Turing Machines and Undecidability has 30 GATE CSE PYQs across 20 papers from 2012 to 2026. 13 of them are 1-mark MCQs like this one, and GATE CSE 2016 Set 2 has 2.

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GateAI solution cardGATE CSE 2016 Set 2 · Q18 · MCQ

Consider the following types of languages: L1 : Regular, L2 : Context-free, L3 : Recursive, L4 : Recursively enumerable. Which of the following is/are TRUE? I. L3 ∪ L4 is recursively enumerable II. L2 ∪ L3 is recursive III. L1* ∩ L2 is context-free IV. L1 ∪ L2 is context-free

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.