Forcing axioms and the complexity of non-stationary ideals.

We study the influence of strong forcing axioms on the complexity of the non-stationary ideal on ω 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 ω 2 to sets of ordinals of countable cofinality is Δ 1 -definable by formulas with parameters in H ( ω 3 ) . The techniques developed in the proof of this result also allow us to prove analogous results for the full non-stationary ideal on ω 2 and strong forcing axioms that are compatible with CH . Finally, we answer a question of S. Friedman, Wu and Zdomskyy by showing that the Δ 1 -definability of the non-stationary ideal on ω 2 is compatible with arbitrary large values of the continuum function at ω 2 .