Christian Michaux, Roger Villemaire, Lecture Notes in Computer Science, 700, Automata, Languages and Programming, 1993, 325
Roger Villemaire, “The theory of is undecidable”, Theoretical Computer Science, 106:2 (1992), 337
Roger Villemaire, Lecture Notes in Computer Science, 577, STACS 92, 1992, 83
А. Л. Семёнов, “Логические теории одноместных функций на натуральном ряде”, Изв. АН СССР. Сер. матем., 47:3 (1983), 623–658 ; A. L. Semenov, “Logical theories of one-place functions on the set of natural numbers”, Math. USSR-Izv., 22:3 (1984), 587–618