Abstract
In this article ‡ we show how the universe of HST, Hrbaček set theory (a nonstandard set theory of “external” type, which includes, in particular, the ZFC Replacement and Separation schemata for all formulas in the language containing the membership and standardness predicates, and Saturation for “standard size” families of internal sets, but does not include the Power set axiom) admits a system of subuniverses which keep the Replacement, model Power set and Choice (in fact all of ZFC, with the exception of the Regularity axiom, which indeed is replaced by the Regularity over the internal subuniverse), and also keep as much of Saturation as it is necessary.
This gives sufficient tools to develop the most complicated topics in nonstandard analysis, such as Loeb measures.
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Partially supported by AMS grants in 1993 – 1995 and DFG grants in 1994 – 1995.
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Kanovei, V., Reeken, M. Internal approach to external sets and universes. Stud Logica 56, 293–322 (1996). https://doi.org/10.1007/BF00372770
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DOI: https://doi.org/10.1007/BF00372770