  Semi-entwining structures are proposed as concepts simpler than entwining
structures, yet they are shown to have interesting applications in constructing
intertwining operators and braided algebras, lifting functors, finding
solutions for Yang-Baxter systems, etc. While for entwining structures one can
associate corings, for semi-entwining structures one can associate comodule
algebra structures where the algebra involved is a bialgebra satisfying certain
properties.
