Selected references
- Alan Cobham. The recognition problem for the set of perfect squares. In Conference Record of 1966 Seventh Annual Symposium on Switching and Automata Theory, pages 78-87, Berkeley, California, 26-28 October 1966. IEEE.
- Marvin Minsky and Seymour Papert. Unrecognizable sets of numbers. Journal of the ACM, 13(2):281-286, April 1966.
- Robert W. Ritchie. Finite automata and the set of squares. Journal of the ACM, 10(4):528-531, October 1963.
- Marcel Paul Schützenberger. A remark on acceptable sets of numbers. Journal of the ACM, 15(2):300-303, April 1968.
- R. E. Stearns, J. Hartmanis, and P. M. Lewis II. Hierarchies of memory limited computations. In Proceedings of the Sixth Annual Symposium on Switching Circuit Theory and Logical Design, pages 179-190. IEEE, 1965.