CONSERVATIVITY OF ULTRAFILTERS OVER SUBSYSTEMS OF SECOND ORDER ARITHMETIC

We extend the usual language of second order arithmetic to one in which we can discuss an ultrafilter over of the sets of a given model. The semantics are based on fixing a subclass of the sets in a structure for the basic language that corresponds to the intended ultrafilter. In this language we state axioms that express the notion that the subclass is an ultrafilter and additional ones that say it is idempotent or Ramsey. The axioms for idempotent ultrafilters prove, for example, Hindman’s theorem and its generalizations such as the Galvin--Glazer theorem and iterated versions of these theorems (IHT and IGG). We prove that adding these axioms to IHT produce conservative extensions of ACA0+IHT,${\rm{ACA}}_{\rm{0}}^ +$, ATR0,${\rm{\Pi }}_2^1$-CA0, and${\rm{\Pi }}_2^1$-CA0for all sentences of second order arithmetic and for full Z2for the class of${\rm{\Pi }}_4^1$sentences. We also generalize and strengthen a metamathematical result of Wang (1984) to show, for example, that any${\rm{\Pi }}_2^1$theorem ∀X∃YΘ(X,Y) provable in ACA0or${\rm{ACA}}_{\rm{0}}^ +$there aree,k∈ ℕ such that ACA0or${\rm{ACA}}_{\rm{0}}^ +$proves that ∀X(Θ(X, Φe(J(k)(X))) where Φeis theeth Turing reduction andJ(k)is thekth iterate of the Turing or Arithmetic jump, respectively. (A similar result is derived for${\rm{\Pi }}_3^1$theorems of${\rm{\Pi }}_1^1$-CA0and the hyperjump.)