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GATE CSE 2021 Set 2 Q19: Language that always has a minimal DFA with the same k states

Question

Let L⊆{0,1}∗L \subseteq \{0,1\}^* be an arbitrary regular language accepted by a minimal DFA with kk states. Which one of the following languages must necessarily be accepted by a minimal DFA with kk states?

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Answer: C (official GATE 2021 final answer key)

Concept used: Complementing a complete DFA only swaps accepting and non-accepting states, so the number of states of the minimal DFA is unchanged.

First step: Swap final and non-final states of the minimal DFA for LL and ask whether any two states could become equivalent.

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Source: official GATE 2021 Set 2 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key