Buss, S.R.; Hay, L. (1988), „On truth-table reducibility to SAT and the difference hierarchy over NP”, Proceedings of Third Annual Structure in Complexity Theory Conference, стр. 224—233, doi:10.1109/SCT.1988.5282
Aho, A. V. (2011), „Complexity theory”, Ур.: Blum, E. K.; Aho, A. V., Computer Science: The Hardware, Software and Heart of It, стр. 241—267, doi:10.1007/978-1-4614-1168-0_12. See in particular pp. 255.