Averages along polynomial sequences in discrete nilpotent groups:   singular Radon transforms
  We consider a class of operators defined by taking averages along polynomial sequences in discrete nilpotent groups. In this paper we prove $L^2$ boundedness of discrete singular Radon transforms along general polynomial sequences in discrete nilpotent groups of step 2. 
Alexandru D. Ionescu, Akos Magyar, and Stephen Wainger
math.CA