Combinatorial Properties of the Raisonnier Filter

The Raisonnier Filter is a combinatorial object isolated by Jean Raisonnier in order to simplify Shelah’s proof that if all ${\mathbf{\Sigma }}_3^1$ sets are Lebesgue-measurable then there is an inner model with an inaccessible cardinal. In this paper, we study the combinatorics of a general version of the Raisonnier filter, with an eye to potential applications in descriptive set theory. Among the most interesting of our results is a partial converse to Raisonnier’s theorem, which can be used to provide a new characterisation of the statement “all ${\mathbf{\Sigma }}_2^1$ sets are measurable”. We also introduce an ideal on the Cantor Space induced by the Raisonnier filter and study its cardinal characteristics, connecting them to the well-known characteristics in Cichoń’s Diagram.