Abstract
We axiomatize hereditarily finite sets in constructive type theory and show that all models of the axiomatization are isomorphic. The axiomatization takes the empty set and adjunction as primitives and comes with a strong induction principle. Based on the axiomatization, we construct the set operations of ZF and develop the basic theory of finite ordinals and cardinality. We construct a model of the axiomatization as a quotient of an inductive type of binary trees. The development is carried out in Coq.
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Notes
- 1.
An impredicative definition of R looks as follows: \(\lambda (XY:\mathrm {HF})(x:X)(y:Y). \forall S:X\rightarrow Y\rightarrow \mathsf {Prop}.~~ S\emptyset \emptyset \rightarrow (\forall axby.~Sab\rightarrow Sxy\rightarrow S(a{.}x)(b{.}y))\rightarrow Sxy \).
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Acknowledgement
Denis Müller contributed to the study of tree equivalence during his Bachelor’s thesis project on finitary sets.
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Smolka, G., Stark, K. (2016). Hereditarily Finite Sets in Constructive Type Theory. In: Blanchette, J., Merz, S. (eds) Interactive Theorem Proving. ITP 2016. Lecture Notes in Computer Science(), vol 9807. Springer, Cham. https://doi.org/10.1007/978-3-319-43144-4_23
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DOI: https://doi.org/10.1007/978-3-319-43144-4_23
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