A note on derivability conditions
Taishi Kurahashi, 倉橋, 太志, クラハシ, タイシ · Institutional Repositories DataBase (IRDB) · 2020
We investigate relationships between versions of derivability conditions for provability predicates. We show several implications and nonimplications between the conditions, and we discuss unprovability of consistency statements induced by derivability conditions. First, we classify already known versions of the second incompleteness theorem, and exhibit some new sets of conditions which are sufficient for unprovability of Hilbert–Bernays’ consistency statement. Secondly, we improve Buchholz’s schematic proof of provable Σ1-completeness. Then among other things, we show that Hilbert–Bernays’ conditions and L¨ob’s conditions are mutually incomparable. We also show that neither Hilbert–Bernays’ conditions nor L¨ob’s conditions accomplish G¨odel’s original statement of the second incompleteness theorem.