The Myhill–Nerode theorem is a fundamental result in formal language theory that provides a characterization of regular languages in terms of equivalence relations on strings. It offers a method to determine whether a language is regular and to construct the minimal deterministic finite automaton (DFA) that recognizes a given regular language.
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A language is regular exactly when indistinguishability by all suffixes has finitely many equivalence classes; these classes form its minimal DFA.