Title: Recognizing Hopf algebroids defined by a group action
Author: Ethan Devinatz
e-mail: devinatz@math.washington.edu
Abstract: Let A be a complete noetherian regular local ring, and suppose
that S is a profinite group acting continuously on A via ring homomorphisms.
Let T be the algebra of continuous functions from S to A. Then (A,T)
has a canonical structure of a complete Hopf algebroid, determined by the
action of S on A. We give necessary and sufficient conditions for a general
Hopf algebroid to be of this form. Applications to Morava theory are also
discussed.