01376nas a2200133 4500008004100000022001400041245003800055210003800093300001200131490000800143520102100151100002001172856005001192 2013 eng d a0168-007200aOperational closure and stability0 aOperational closure and stability a813-8210 v1643 aIn this article we introduce and study the notion of operational closure: a transitive set d is called operationally closed iff it contains all constants of $\mathsf{OST}$ and any operation $f\in d$ applied to an element $a\in d$ yields an element $fa\in d$ , provided that $f$ applied to a has a value at all. We will show that there is a direct relationship between operational closure and stability in the sense that operationally closed sets behave like $\Sigma_1$ substructures of the universe. This leads to our final result that $\mathsf{OST}$ plus the axiom $(\mathsf{OLim})$ , claiming that any set is element of an operationally closed set, is proof-theoretically equivalent to the system $\mathsf{KP}+(\Sigma_1\text{-}\mathsf{Sep})$ of Kripke-Platek set theory with infinity and $\Sigma_1$ separation. We also characterize the system $\mathsf{OST}$ plus the existence of one operationally closed set in terms of Kripke-Platek set theory with infinity and a parameter-free version of $\Sigma_1$ separation.1 aJäger, Gerhard uhttp://www.iam.unibe.ch/ltgpub/2013/jae13.pdf