We've been learning the arithmetical hierarchy, but unfortunately the parts of the textbook used actually did not include the formal definitions of a $\Delta$ class (we are being provided with parts of a pdf). I was wondering if anyone could provide one. Thank you.
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$\Delta_n^0 = \Sigma_n^0 \cap \Pi_n^0$. – Hayden May 03 '17 at 17:40
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@Hayden As in a set that can be expressed as both $\Sigma_n$ and $\Pi_n$? – layabout May 03 '17 at 18:01
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Yep, exactly ${}{}{}{}$ – Hayden May 03 '17 at 18:07