Maximal set (computability theory): Difference between revisions

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In [[recursioncomputability theory]], the [[mathematics|mathematical]] theory of computability, a '''maximal set''' is a coinfinite [[recursively enumerable set|recursively enumerable subset]] ''A'' of the [[natural number]]s such that for every further recursively enumerable subset ''B'' of the natural numbers, either ''B'' is [[cofinite]] or ''B'' is a finite variant of ''A'' or ''B'' is not a superset of ''A''. This gives an easy definition within the [[lattice (order)|lattice]] of the recursively enumerable sets.

Maximal sets have many interesting properties: they are [[simple set|simple]], [[hypersimple set|hypersimple]], [[hyperhypersimple]] and r-maximal; the latter property says that every recursive set ''R'' contains either only finitely many elements of the complement of ''A'' or almost all elements of the complement of ''A''. There are r-maximal sets that are not maximal; some of them do even not have maximal supersets. Myhill (1956) asked whether maximal sets exist and Friedberg (1958) constructed one. Soare (1974) showed that the maximal sets form an orbit with respect to [[automorphism]] of the recursively enumerable sets under inclusion ([[Modulo (jargon)|modulo]] finite sets). On the one hand, every automorphism maps a maximal set ''A'' to another maximal set ''B''; on the other hand, for every two maximal sets ''A'', ''B'' there is an automorphism of the recursively enumerable sets such that ''A'' is mapped to ''B''.