On a factorization of graded Hopf algebras using Lyndon words
  We find a generalization of the restricted PBW basis for pointed Hopf algebras over abelian groups constructed by Kharchenko. We obtain a factorization of the Hilbert series for a wide class of graded Hopf algebras. These factors are parametrized by Lyndon words, and they are the Hilbert series of certain graded Hopf algebras.   Key words: graded Hopf algebra, Nichols algebra, Lyndon word 
M. Gra\~na, I. Heckenberger