Leonid Levin (1973). „Universal'nye perebornye zadachi”. Problemy Peredachi Informatsii9 (3): 265–266. English translation, "Universal Search Problems", in B. A. Trakhtenbrot (1984). „A Survey of Russian Approaches to Perebor (Brute-Force Searches) Algorithms”. Annals of the History of Computing6 (4): 384-400.