1. Ash, C.J., Knight, J.F.: Computable Structures and the Hyperarithmetical Hierarchy, #144 in Studies in Logic. Elsevier Science, Amsterdam (2000).
2. Baldwin, J.: Amalgamation, absoluteness, and categoricity. In Proceedings of the 11th Asian Logic Conference 2009, World Scientific, Singapore (2010)
3. Baldwin, J., Larson, P.: Iterated Elementary Embeddings and the Model Theory of Infinitary Logic (pre-print)
4. Barwise K.J.: Infinitary logic and admissible sets. J. Symb. Logic 34, 226–252 (1969)
5. Friedman S.D.: β-recursion theory. Trans. AMS 255, 173–200 (1979)