Forcing axioms and the complexity of non-stationary ideals

TL;DR

This paper investigates how strong forcing axioms influence the definability complexity of the non-stationary ideal on 92, showing certain definability properties are independent of these axioms and compatible with various continuum sizes.

Contribution

It demonstrates that the strengthening $MM^{++}$ does not determine the definability of the non-stationary ideal on 92, and extends results to axioms compatible with CH and large continuum values.

Findings

$MM^{++}$ does not decide the $ riangle_1$-definability of the non-stationary ideal.

The techniques apply to the full non-stationary ideal and axioms compatible with CH.

The $ riangle_1$-definability is compatible with arbitrarily large continuum at 92.

Abstract

We study the influence of strong forcing axioms on the complexity of the non-stationary ideal on $\omega_2$ and its restrictions to certain cofinalities. Our main result shows that the strengthening $MM^{++}$ of Martin's Maximum does not decide whether the restriction of the non-stationary ideal on $\omega_2$ to sets of ordinals of countable cofinality is $\Delta_1$-definable by formulas with parameters in $H(\omega_3)$. The techniques developed in the proof of this result also allow us to prove analogous results for the full non-stationary ideal on $\omega_2$ and strong forcing axioms that are compatible with CH. Finally, we answer a question of S. Friedman, Wu and Zdomskyyshow by showing that the $\Delta_1$-definability of the non-stationary ideal on $\omega_2$ is compatible with arbitrary large values of the continuum function at $\omega_2$.