Melnikov A.G.; Selivanov V.L.; Yamaleev M.M.
(2020)
© 2019 Elsevier B.V. In the late 1980s, Selivanov used typed Boolean combinations of arithmetical sets to extend the Ershov hierarchy beyond Δ20 sets. Similar to the Ershov hierarchy, Selivanov's fine hierarchy {Σγ}γ<ε0 ...