Gc. Rota et Ja. Stein, PLETHYSTIC HOPF-ALGEBRAS, Proceedings of the National Academy of Sciences of the United Statesof America, 91(26), 1994, pp. 13057-13061
The notion of a plethystic algebra associated with a Hopf algebra endo
wed with a suitable bilinear form is defined. A special case is the Ho
pf algebra of symmetric functions.