A word Hopf algebra based on the selection/quotient principle
  In this paper, we define a Hopf algebra structure on the vector space spanned by packed words using a selection/quotient coproduct. We show that this algebra is free on its irreducible packed words. We also construct the Hilbert series of this Hopf algebra and we investigate its primitive elements. 
G. H. E. Duchamp, N. Hoang-Nghia and A. Tanasa
math.CO hep-th