Approximate substitutions and the normal ordering problem
  In this paper, we show that the infinite generalised Stirling matrices associated with boson strings with one annihilation operator are projective limits of approximate substitutions, the latter being characterised by a finite set of algebraic equations. 
H. Cheballah (LIPN), G. H. E. Duchamp (LIPN), K. A. Penson (LPTMC)