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.