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when toggle format what by license comment
Jan 2, 2018 at 14:28 comment added David E Speyer @Libli See en.wikipedia.org/wiki/Arithmetical_hierarchy
Jan 2, 2018 at 14:01 comment added Libli what are $\Pi_{2}^0$ and $\Pi_{3}^0$?
Jan 2, 2018 at 11:07 history asked Alex Gavrilov CC BY-SA 3.0