Hopf Algebras and Galois Extensions of an Algebra H. KreimerM. Takeuchi Indiana University Mathematics Journal 1981 text pdf 10.1512/iumj.1981.30.30052 10.1512/iumj.1981.30.30052 en Indiana Univ. Math. J. 30 (1981) 675 - 692 state-of-the-art mathematics http://iumj.org/access/