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Turing Machines and Undecidability

GATE CSE 2023 · Q24

MSQ1 markEasy

Time

Which of the following statements is/are CORRECT?

Select all that applyOne or more correct

Pick every correct option

Solution

Step-by-step verified solution

Official answer: A) The intersection of two regular languages is regular.; C) The intersection of two recursive languages is recursive.; D) The intersection of two recursively enumerable languages is recursively enumerable.

Turing Machines and Undecidability

Let us check the closure properties under intersection for each class of languages:

  • (A) Regular languages: Regular languages are closed under intersection. If L1L_1 and L2L_2 are regular, their intersection L1∩L2L_1 \cap L_2 can be accepted by a product automaton, or by De Morgan's Law: L1∩L2=L1‾∪L2‾‾L_1 \cap L_2 = \overline{\overline{L_1} \cup \overline{L_2}}. Thus, statement (A) is CORRECT.

  • (B) Context-free languages (CFLs): CFLs are not closed under intersection. For example, L1={anbncm∣n,m≥0}L_1 = \{a^n b^n c^m \mid n, m \ge 0\} and L2={ambncn∣m,n≥0}L_2 = \{a^m b^n c^n \mid m, n \ge 0\} are both context-free, but their intersection L1∩L2={anbncn∣n≥0}L_1 \cap L_2 = \{a^n b^n c^n \mid n \ge 0\} is not context-free. Thus, statement (B) is INCORRECT.

  • (C) Recursive languages (Decidable): Recursive languages are closed under intersection. Given Turing machines (deciders) M1M_1 and M2M_2 for L1L_1 and L2L_2, a decider for L1∩L2L_1 \cap L_2 runs M1M_1 on input ww; if M1M_1 accepts, it runs M2M_2 on ww, accepting if and only if both accept. Since both deciders always halt, the composite machine also always halts. Thus, statement (C) is CORRECT.

  • (D) Recursively enumerable languages (Turing-recognizable): Recursively enumerable languages are closed under intersection. Given Turing machines M1M_1 and M2M_2 recognizing L1L_1 and L2L_2, a Turing machine for L1∩L2L_1 \cap L_2 runs M1M_1 on input ww, and if M1M_1 accepts, it then runs M2M_2 on ww (or runs them in parallel). It accepts if both accept. Thus, statement (D) is CORRECT.

Hence, the correct options are A, C, and D.

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This Turing Machines and Undecidability previous year question is Q24 of the GATE CSE 2023 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. 3 of them are 1-mark MSQs like this one, and it is the only one in GATE CSE 2023.

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GateAI solution cardGATE CSE 2023 · Q24 · MSQ

Which of the following statements is/are CORRECT?

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