Groupoids, covers, and 3-uniqueness in stable theories

Abstract. Building on Hrushovski's work in [5], we study definable groupoids in stable theories and their relationship with 3-uniqueness and finite internal covers. We introduce the notion of retractability of a definable groupoid (which is slightly stronger than Hrushovski's notion of eliminability), give some criteria for when groupoids are retractable, and show how retractability relates to both 3-uniqueness and the splitness of finite internal covers. One application we give is a new direct method of constructing noneliminable groupoids from witnesses to the failure of 3-uniqueness. Another application is a proof that any finite internal cover of a stable theory with a centerless liaison groupoid is almost split.

• Type: Journal
• Year: 2010
• Where: JSYML
• Authors: John Goodrick, Alexei Kolesnikov
• Keywords: Definable Groupoid, Finite Internal Cover, Groupoid, JSYML 2010