Duality for Ext-groups and extensions of discrete series for graded   Hecke algebras
  In this paper, we study extensions of graded affine Hecke algebra modules. In particular, based on an explicit projective resolution on graded affine Hecke algebra modules, we prove a duality result for Ext-groups. This duality result with an Ind-Res resolution gives an algebraic proof of the fact that all higher Ext-groups between discrete series vanish. 
Kei Yuen Chan
math.RT