A product formula and combinatorial field theory
  We treat the problem of normally ordering expressions involving the standard boson operators a, a* where [a,a*]=1. We show that a simple product formula for formal power series - essentially an extension of the Taylor expansion - leads to a double exponential formula which enables a powerful graphical description of the generating functions of the combinatorial sequences associated with such functions - in essence, a combinatorial field theory. We apply these techniques to some examples related to specific physical Hamiltonians. 
A. Horzela (4), P. Blasiak (1,4), G.H.E. Duchamp (3), K.A. Penson (1),   A.I. Solomon (1,2) ((1) LPTL, University of Paris V...