  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

Quantum Algebra