Double Principal Bundles
  We define double principal bundles (DPBs), for which the frame bundle of a double vector bundle, double Lie groups and double homogeneous spaces are basic examples. It is shown that a double vector bundle can be realized as the associated bundle of its frame bundle. Also dual structures, gauge transformations and connections in DPBs are investigated. 
Honglei Lang, Yanpeng Li and Zhangju Liu
math.DG math-ph math.MP