Journal of Symbolic Logic
- J. Symbolic Logic
- Volume 64, Issue 2 (1999), 769-774.
Ordinal Inequalities, Transfinite Induction, and Reverse Mathematics
If $\alpha$ and $\beta$ are ordinals, $\alpha \leq \beta$, and $\beta \nleq \alpha$, then $\alpha + 1 \leq \beta$. The first result of this paper shows that the restriction of this statement to countable well orderings is provably equivalent to ACA$_0$, a subsystem of second order arithmetic introduced by Friedman. The proof of the equivalence is reminiscent of Dekker's construction of a hypersimple set. An application of the theorem yields the equivalence of the set comprehension scheme ACA$_0$ and an arithmetical transfinite induction scheme.
J. Symbolic Logic, Volume 64, Issue 2 (1999), 769-774.
First available in Project Euclid: 6 July 2007
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Hirst, Jeffry L. Ordinal Inequalities, Transfinite Induction, and Reverse Mathematics. J. Symbolic Logic 64 (1999), no. 2, 769--774. https://projecteuclid.org/euclid.jsl/1183745808