1. Davis, M.: Engines of Logic: Mathematicians and the Origin of the Computer. Norton (2001)
2. Börger, E., Grädel, E., Gurevich, Y.: The Classical Decision Problem. Springer, Heidelberg (1996)
3. Dreben, D., Goldfarb, W.D.: The Decision Problem: Solvable Classes of Quantificational Formulas. Addison-Wesley, Reading (1979)
4. Löwenheim, L.: Über Möglichkeiten im Relativkalküll (On possibilities in the claculus of relations). Math. Ann. 76, 447–470 (1915); Translated in From Frege to Gödel, van Heijenoort. Harvard Univ. Press (1971)
5. Skolem, T.: Untersuchung über Axiome des Klassenkalküls und über Produktations- und Summationsprobleme, welche gewisse Klassen von Aussagen betreffen (Investigations of the axioms of the calculusof classes and on product and sum problems that are connected with certain class of statements). Videnskabsakademiet i Kristiania, Skrifter I 3 (1919);