On the Hopf algebraic origin of Wick normal-ordering
  A combinatorial formula of G.-C. Rota and J.A. Stein is taken to perform Wick re-ordering in quantum field theory. Wick's theorem becomes a Hopf algebraic identity called Cliffordization. The combinatorial method relying on Hopf algebras is highly efficient in computations and yields closed algebraic expressions.   AMS Subject Classifications 2000: 81R50; 16W30 
Bertfried Fauser