1. Bachmann, H., Transfinite Zahlen, Springer, Berlin (1955)
2. Barwise, K. J., Infinitary logic and admissible sets, Dissertation, Stanford University (1967).
3. Beth, E. W., On Padoa's method in the theory of definition Indag. Math., v. 15 (1953), pp. 330–339.
4. Choodnovsky, G., Some results in theory of infinitely long expressions, (abstract) Proc. 3d Intl. Cong. for Logic, Methodology and Philos. of Science, Amsterdam, 1967 (to appear).
5. Craig, W., Linear reasoning. A new form of the Herbrand-Gentzen theorem, Journ. Symbolic Logic, v. 22 (1957), pp. 250–268.