SET IDEALS WITH COMPLETE SYMMETRY GROUP AND PARTITION IDEALS

Abstract

For a wide class of set ideals (including, for example, all uniform ideals), a criterion of completeness of their symmetry groups is provided in terms of ideal quotients (polars). We apply it to partition ideals, and derive the extended Sierpiński–Erdös duality principle. We demonstrate that if the measure and category ideals I₀ and I₁ on the real line ℝ are partition (or, equivalently, if just I₀ is partition), then not only are they isomorphic via an involution, but they also have complete (and distinct) symmetry groups coinciding, respectively, with the symmetry groups of the polars I^⊥₀ and I^⊥₁. The measure and category ideals on ℝ (and in more general spaces) are partition (Oxtoby) ideals assuming Martin's Axiom. In this case their polars are, respectively, the ideals generated by c-Sierpiński and c-Lusin sets. It is well known that the isomorphism of the measure and category ideals is not provable in ZFC. We show that the isomorphism of their symmetry groups is likewise unprovable.