Mapping sets and hypersets into numbers. (English) Zbl 1371.03060
Summary: We introduce and prove the basic properties of encodings that generalize to non-well-founded hereditarily finite sets the bijection defined by W. Ackermann [Math. Ann. 114, 305–315 (1937; Zbl 0016.19501)] between hereditarily finite sets and natural numbers.
MSC:
| 03E30 | Axiomatics of classical set theory and its fragments |
| 03E70 | Nonclassical and second-order set theories |
| 03E65 | Other set-theoretic hypotheses and axioms |
