Harvey Friedman (1975, 1976) Friedman, Harvey (1975), "Some systems of second-order arithmetic and their use", Proceedings of the International Congress of Mathematicians (Vancouver, B. C., 1974), Vol. 1, Montreal: Canad. Math. Congress, pp. 235–242, MR0429508 Friedman, Harvey (1976), Baldwin, John; Martin, D. A.; Soare, R. I.; Tait, W. W. (eds.), "Systems of second-order arithmetic with restricted induction, I, II", Meeting of the Association for Symbolic Logic, The Journal of Symbolic Logic, 41 (2): 557–559, doi:10.2307/2272259, JSTOR2272259
Kołodziejczyk, Leszek; Michalewski, Henryk (2016). How unprovable is Rabin's decidability theorem?. LICS '16: 31st Annual ACM/IEEE Symposium on Logic in Computer Science. arXiv:1508.06780.
Harvey Friedman (1975, 1976) Friedman, Harvey (1975), "Some systems of second-order arithmetic and their use", Proceedings of the International Congress of Mathematicians (Vancouver, B. C., 1974), Vol. 1, Montreal: Canad. Math. Congress, pp. 235–242, MR0429508 Friedman, Harvey (1976), Baldwin, John; Martin, D. A.; Soare, R. I.; Tait, W. W. (eds.), "Systems of second-order arithmetic with restricted induction, I, II", Meeting of the Association for Symbolic Logic, The Journal of Symbolic Logic, 41 (2): 557–559, doi:10.2307/2272259, JSTOR2272259
Harvey Friedman (1975, 1976) Friedman, Harvey (1975), "Some systems of second-order arithmetic and their use", Proceedings of the International Congress of Mathematicians (Vancouver, B. C., 1974), Vol. 1, Montreal: Canad. Math. Congress, pp. 235–242, MR0429508 Friedman, Harvey (1976), Baldwin, John; Martin, D. A.; Soare, R. I.; Tait, W. W. (eds.), "Systems of second-order arithmetic with restricted induction, I, II", Meeting of the Association for Symbolic Logic, The Journal of Symbolic Logic, 41 (2): 557–559, doi:10.2307/2272259, JSTOR2272259
Simpson claims to have not invented the term. [Simpson, S.; Eastaugh, B.; Dean, W. (June 17, 2022). "Panel Discussion". YouTube. Paris, France: University of Chicago, Reverse Mathematics and its Philosophy.]