In this paper we introduce a notation system for the infinitary
derivations occurring in the ordinal analysis of
KP+Π_3-Reflection due to Michael Rathjen. This allows a
finitary ordinal analysis of
KP+Π₃-Reflection. The method used is an
extension of techniques developed by Wilfried Buchholz, namely
operator controlled notation systems for
RS∞-derivations. Similarly to Buchholz we obtain a
characterisation of the provably recursive functions of
KP+Π₃-Reflection as <-recursive
functions where < is the ordering on Rathjen’s
ordinal notation system 𝒯(K). Further we show a conservation
result for Π⁰₂-sentences.
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