1. Barendregt, H.P.: Problem #26 in a list of open problems maintained by the università di torino. http://tlca.di.unito.it/opltlca/opltlcasu33.html. Accessed 20 Apr 2022
2. Curry, H.B., Hindley, J.R., Seldin, J.P.: Combinatory Logic, vol. 2. North-Holland, Amsterdam (1972)
3. Gentzen, G.: Neue Fassung des Widerspruchsfreiheitsbeweises für die reine Zahlentheorie. Hirzel, Leipzig (1938)
4. Hindley, J.R.: Axioms for strong reduction in combinatory logic. J. Symb. Log. 32(2), 224–236 (1967)
5. Hindley, J.R., Seldin, J.P.: Lambda Calculus and Combinators: An Introduction. Cambridge University Press, Cambridge (2008)