Σ1-elementarity and Skolem hull operators

AbstractThe exact correspondence between ordinal notations derived from Skolem hull operators, which are classical in ordinal analysis, and descriptions of ordinals in terms of Σ1-elementarity, an approach developed by T.J. Carlson, is analyzed in full detail. The ordinal arithmetical tools needed for this purpose were developed in [G. Wilken, Ordinal arithmetic based on Skolem hulling, Annals of Pure and Applied Logic 145 (2) (2007) 130–161].We show that the least ordinal κ such that κ<1∞ (as defined in [T.J. Carlson, Elementary patterns of resemblance, Annals of Pure and Applied Logic 108 (2001) 19–77] and described below) is the proof theoretic ordinal of the set-theoretic system KPℓ0, confirming a claim of Carlson. Moreover, we characterize the class of all ordinals κ such that κ