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Regular Expressions and Finite Automata

GATE CSE 2013 · Q33

MCQ2 marksMedium

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Consider the DFA A given below. Which of the following are FALSE? 1. Complement of L(A)L(A) is context-free. 2. L(A)=L((11∗0+0)(0+1)∗0∗1∗)L(A) = L((11^*0+0)(0 + 1)^*0^*1^*) 3. For the language accepted by AA, AA is the minimal DFA. 4. AA accepts all strings over {0,1}\{0, 1\} of length at least 2.

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Solution

Step-by-step verified solution

Official answer: D) 3 and 4 only

Regular Expressions and Finite Automata

Let the states of DFA AA be q0q_0 (initial state, top-left), q1q_1 (top-right), and q2q_2 (accepting state, bottom).

  • From q0q_0, on input 0 it transitions directly to q2q_2.

  • From q0q_0, on input 1 it transitions to q1q_1. From q1q_1, any sequence of 1's loops at q1q_1, and on the first 0, it transitions to q2q_2.

  • Once in q2q_2, it loops on both 0 and 1 (trap/absorbing accept state).

Thus, L(A)L(A) accepts all strings that contain at least one 0:

L(A)=(0+1)∗0(0+1)∗L(A) = (0 + 1)^*0(0 + 1)^*

Now evaluate each statement:

  1. Statement 1: L(A)L(A) is regular, so its complement is also regular and therefore context-free. (TRUE)

  2. Statement 2: (11∗0+0)(0+1)∗0∗1∗=(1+0+0)(0+1)∗=1∗0(0+1)∗=(0+1)∗0(0+1)∗=L(A)(11^*0 + 0)(0+1)^*0^*1^* = (1^+0 + 0)(0+1)^* = 1^*0(0+1)^* = (0+1)^*0(0+1)^* = L(A). (TRUE)

  3. Statement 3: The language L(A)=(0+1)∗0(0+1)∗L(A) = (0+1)^*0(0+1)^* can be accepted by a 2-state minimal DFA (state p0p_0 loops on 1 and goes to p1p_1 on 0; p1p_1 is accepting and loops on 0, 1). Thus, AA (which has 3 states) is not minimal. (FALSE)

  4. Statement 4: String 11 has length 2 and only reaches q1q_1 (non-accepting). So AA does not accept all strings of length at least 2. (FALSE)

Therefore, statements 3 and 4 are FALSE, which corresponds to option (D).

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This Regular Expressions and Finite Automata previous year question is Q33 of the GATE CSE 2013 paper. Find more GATE CSE PYQs on Regular Expressions and Finite Automata in Theory of Computation, or browse all GATE CSE PYQs.

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Regular Expressions and Finite Automata has 41 GATE CSE PYQs across 24 papers from 2011 to 2026. 10 of them are 2-mark MCQs like this one, and GATE CSE 2013 has 2.

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GateAI solution cardGATE CSE 2013 · Q33 · MCQ

Consider the DFA A given below. Which of the following are FALSE? 1. Complement of L(A) is context-free. 2. L(A) = L((11*0+0)(0 + 1)*0*1*) 3. For the language accepted by A, A is the minimal DFA. 4. A accepts all strings over {0, 1} of length at least 2.

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