I[omega_2] can be the nonstationary ideal on Cof(omega_1)
arXiv:math/0407225
Abstract
We answer a question of Shelah by showing that it is consistent that every set of ordinals of cofinality omega_1 in I[omega_2] is nonstationary if and only if it is consistent that that there is a kappa^+ Mahlo cardinal kappa.
version 4 of the paper has been extensively revised in response to the referee report from TAMS. I expect it to appear in TAMS