1. Barendregt, The Lambda Calculus, North Holland, 1984.
2. Berrarducci and Intrigila, Some new results on easy lambda terms in Dezani-Ciancaglini, Ronchi Della Rocha, and Venturini Zilli, A Collection of Contributions in Honor Corrado Bohm, Elsevier, 1993.
3. Lect Notes Comput Sci;Jacopini,1975
4. Statman, On sets of solutions to combinator equations, TCS, 66, p. 99–104.
5. Statman, Morphism and partitions of V-sets, Carnegie Mellon University, Department of Mathematical Sciences Research Report 97-210, September, 1997.