Abstract

In continuation of our study of HST, Hrbacek set theory (a nonstandard set theory which includes, in particular, the ZFC Replacement and Separation schemata in the st-∈-language, and Saturation for well-orderable families of internal sets), we consider the problem of existence of elementary extensions of inner "external" subclasses of the HST universe. We show that, given a standard cardinal κ, any set R ⊑ *κ generates an "internal" class S(R) of all sets standard relatively to elements of R, and an "external" class L[S(R)] of all sets constructible (in a sense close to the Godel constructibility) from sets in S(R). We prove that under some mild saturation-like requirements for R the class L[S(R)] models a certain κ-version of HST including the principle of κ+-saturation; moreover, in this case L[S(R′)] is an elementary extension of L[S(R)] in the st-∈-language whenever sets R ⊑ R′ satisfy the requirements.

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