Inner Models from Extended Logics: Part 1

TL;DR

This paper explores inner models derived from extended logics, showing their robustness and introducing a new model based on the cofinality quantifier that exhibits unique properties under large cardinal assumptions.

Contribution

It introduces a new inner model from the cofinality quantifier and demonstrates its properties and robustness compared to classical models like L and HOD.

Findings

L and HOD are insensitive to the choice of logic

The cofinality quantifier yields a new inner model between L and HOD

Under large cardinals, the model's properties relate to weakly compact cardinals and forcing absoluteness

Abstract

If we replace first order logic by second order logic in the original definition of G\"odel's inner model $L$, we obtain HOD. In this paper we consider inner models that arise if we replace first order logic by a logic that has some, but not all, of the strength of second order logic. Typical examples are the extensions of first order logic by generalized quantifiers, such as the Magidor-Malitz quantifier, the cofinality quantifier, or stationary logic. Our first set of results show that both $L$ and HOD manifest some amount of {\em formalism freeness} in the sense that they are not very sensitive to the choice of the underlying logic. Our second set of results shows that the cofinality quantifier gives rise to a new robust inner model between $L$ and HOD. We show, among other things, that assuming a proper class of Woodin cardinals the regular cardinals $>\aleph_1$ of $V$ are weakly compact in the inner model arising from the cofinality quantifier and the theory of that model is (set) forcing absolute and independent of the cofinality in question. We do not know whether this model satisfies the Continuum Hypothesis, assuming large cardinals, but we can show, assuming three Woodin cardinals and a measurable above them, that if the construction is relativized to a real, then on a cone of reals the Continuum Hypothesis is true in the relativized model.