1. Ash CJ, Knight JF (2000) Computable structures and the hyperarithmetical hierarchy. North Holland, Amsterdam
2. Chang CC, Keisler HJ (1990) Model theory. North-Holland, Amsterdam
3. Bell JL, Slomson AB (1974) Models and ultraproducts: an introduction, 3rd revised printing, North-Holland, Amsterdam
4. Dekker JCE, Myhill J (1960) Recursive equivalence types. University of California Publications in Mathematics, N.S., vol 3, pp 67–213
5. Dimitrov R, Harizanov V, Miller R, Mourad KJ (2014) Isomorphisms of non-standard fields and Ash’s conjecture, 10th Conference on Computability in Europe. In: Beckmann A, Csuhaj-Varjú E, Meer K (eds) Language, life, limits, Lecture Notes in Computer Science 8493. Springer, pp 143–152