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Nov 21, 2015 at 19:19 comment added Denis Hirschfeldt @Andreas Blass I think this is a slightly tricky theorem to credit. The paper you cite proves that if a set is c.e. relative to every real in a class of positive measure, then it is c.e. Of course, the analogous result for computable in place of c.e. is a one-line corollary, but I believe it was never stated in that paper. Later Sacks stated it explicitly and proved it, with essentially the same argument, though as far as I know without knowledge of the earlier paper.
Nov 21, 2015 at 15:06 comment added Carl Mummert Thanks, Andreas. I had somehow learned the theorem as just due to Sacks. Fortunately Downey and Hirschfeldt state both versions.
Nov 21, 2015 at 15:05 history edited Carl Mummert CC BY-SA 3.0
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Nov 21, 2015 at 11:13 comment added Andreas Blass The fact that, if a real is computable from every real in a set of positive measure, then it's computable is, as far as I know, proved, though with rather different teminology, in the paper (citation opied from MathSciNet): de Leeuw, K.; Moore, E. F.; Shannon, C. E.; Shapiro, N. Computability by probabilistic machines. Automata studies, pp. 183–212. Annals of mathematics studies, no. 34. Princeton University Press, Princeton, N. J., 1956.
Nov 21, 2015 at 0:12 vote accept Abram Demski
Nov 21, 2015 at 0:29
Nov 21, 2015 at 0:09 history edited Carl Mummert CC BY-SA 3.0
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Nov 20, 2015 at 23:59 history answered Carl Mummert CC BY-SA 3.0