1. R. G. Downey and D. R. Hirschfeldt, Algorithmic Randomness and Complexity, Theory and Applications of Computability, Springer, New York, 2010.
2. R. G. Downey, D. R. Hirschfeldt, S. Lempp and R. Solomon, Reverse mathematics of the Nielsen–Schreier theorem, in Proceedings of International Conferences on Mathematical Logic, Novosibirsk State University Press, Novosibirsk, 2002, pp. 59–71.
3. R. G. Downey and A. Melnikov, Effectively categorical abelian groups, Journal of Algebra 373 (2013), 223–248.
4. P. C. Eklof, Whitehead’s problem is undecidable, American Mathematics Monthly 83 (1976), 775–788.
5. L. Fuchs, Infinite Abelian Groups. Vol. I, Pure and Applied Mathematics, Vol. 36, Academic Press, New York–London, 1970.