Arrow's Impossibility Theorem and Ultrafilters. References I need some references (far away from Wikipedia) about the proof using Ultrafilters of Arrow's Impossibility Theorem. Online resources are preferred.

Since no one else has answered, I’m turning my comment into an answer. This blog post gives a reasonably clear version of the proof. This one is a bit better and has a nice follow-on on the Gibbard-Satterthwaite theorem. This PDF by Joel W. Robbin at the University of Wisconsin — Madison also does a nice job.