1. Aczel, P.: Describing ordinals using functionals of transfinite type. JSL37, 35–47 (1972)
2. Barwise, J.: Admissible sets and structures. An approach to definability theory. Berlin Heidelberg New York: Springer 1975
3. Buchholz, W., Schütte, K.: Ein Ordinalzahlensystem für die beweistheoretische Abgrenzung derΠ 2 1 -Separation und Bar-Induktion. Bayer. Akad. Wiss. Math.-Naturwiss. Kl. Sitzungsber.1983, 99–132 (1984)
4. Feferman, S.: Ordinals associated with theories for one inductively defined set (preliminary version). Hektographiertes Manuskript eines Beitrags zur Summer Conference at Buffalo N.Y. 1968
5. Feferman, S.: Hereditarily replete functionals over the ordinals. In: Kino, A., Myhill, J., Vesley, R.E. (eds.) Intuitionism and proof theory. Proceedings of the Summer Conference at Buffalo N.Y. 1968, pp. 289–301. Amsterdam London: North-Holland 1970