AbstractWe define what it means for a function on ω1 to be a collapsing function for λ and show that if there exists a collapsing function for (2)+, then there is no precipitous ideal on ω1. We show that a collapsing function for ω2 can be added by forcing. We define what it means to be a weakly ω1‐Erdös cardinal and show that in L[E], there is a collapsing function for λ iff λ is less than the least weakly ω1‐Erdös cardinal. As a corollary to our results and a theorem of Neeman, the existence of a Woodin limit of Woodin cardinals does not imply the existence of precipitous ideals on ω1. We also show that the following statements hold in L[E]. The least cardinal λ with the Chang property (λ, ω1) ↠ (ω1, ω) is equal to the least ω1‐Erdös cardinal. In particular, if j is a generic elementary embedding that arises from non‐stationary tower forcing up to a Woodin cardinal, then the minimum possible value of j(ω1) is the least ω1‐Erdös cardinal. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)