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Unformatted text preview: Harvard CS 121 and CSCI E207 Lecture 16: Reductions Harry Lewis November 5, 2009 Reading: Sipser Ch. 5 Harvard CS 121 & CSCI E207 November 5, 2009 CoX For any property X that a set might have, a set S is coX iff S has property X. For example, a cofinite set of natural numbers is a set that is missing only a finite number of elements. A coregular language is . . . ? A corecursive language is . . . ? What about a coCF language? Proved last time: A language is recursive if and only if it is both r.e. and cor.e. 1 Harvard CS 121 & CSCI E207 November 5, 2009 Nonr.e. Languages Theorem: The following cor.e. languages are not r.e.: A TM = {h M,w i : M does not accept w } HALT TM = {h M,w i : M does not halt on w } HALT TM = {h M i : M does not halt on } Proof: If these languages were r.e., then A TM , HALT TM , and HALT TM would be both r.e. and cor.e. and hence recursive. 2 Harvard CS 121 & CSCI E207 November 5, 2009 Is it possible to determine, given a TM M , whether M accepts a finite or infinite set? Let A infinite = {h M i : L ( M ) is infinite } . Is A infinite recursive?...
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 Fall '09

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