PBW--filtration over $\bz$ and compatible bases for $V_\bz(\la)$ in type   ${\tt A}_n$ and ${\tt C}_n$
  We study the PBW-filtration on the highest weight representations $V(\la)$ of the Lie algebras of type ${\tt A}_{n}$ and ${\tt C}_{n}$. This filtration is induced by the standard degree filtration on $\U(\fn^-)$. In previous papers, the authors studied the filtration and the associated graded algebras and modules over the complex numbers. The aim of this paper is to present a proof of the results which holds over the integers and hence makes the whole construction available over any field. 
Evgeny Feigin, Ghislain Fourier and Peter Littelmann
math.RT