COQUASITRIANGULAR STRUCTURES FOR EXTENSIONS OF HOPF ALGEBRAS. APPLICATIONS

Let A subset of E be an extension of Hopf algebras such that there exists a normal left A-module coalgebra map pi : E -> A that splits the inclusion. We shall describe the set of all coquasitriangular structures on the Hopf algebra E in terms of the datum (A, E, pi) as follows: first, any such extension E is isomorphic to a unified product A proportional to H, for some unitary subcoalgebra H of E (A. L. Agore and G. Militaru, Unified products and split extensions of Hopf algebras, to appear in AMS Contemp. Math.). Then, as a main theorem, we establish a bijective correspondence between the set of all coquasitriangular structures on an arbitrary unified product A proportional to H and a certain set of datum (p, tau, u, v) related to the components of the unified product. As the main application, we derive necessary and sufficient conditions for Majid's infinite-dimensional quantum double D-lambda (A, H) = A (sic)(tau) H to be a coquasitriangular Hopf algebra. Several examples are worked out in detail.