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Unformatted text preview: MyhillNerode Handout Definition. An equivalence relation E on strings is right invariant iff concatenating a string w onto two equivalent strings u and v produces two strings ( uw and vw ) that are also equivalent; i.e., for all strings u , v , and w , we have uE v uw E vw . Theorem 1. A language L is accepted by a DFA iff L is the union of some equivalence classes of a rightinvariant equivalence relation of finite index. Proof, Part A. Suppose that a lan guage L is accepted by a DFA M = h S, ,,s ,F i . Define an equivalence relation E so that two strings u and w are equivalent iff the DFA (starting in state s ) would transition to the same state by reading either u or w , i.e., uE v iff ( s ,u ) = ( s ,v ) Let EC ( s i ) denote the equivalence class { w  ( s ,w ) = s i } (i.e., the set of strings that transition the DFA to state s i ). It is easy to verify that all members of EC ( s i ) are indeed equivalent to each other. E is of finite index, because the set of equivalence classes is...
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 Spring '09
 EdmundM.Clarke

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