Singular Integrals with Flag Kernels on Homogeneous Groups: I
  Let $\mathcal K$ be a flag kernel on a homogeneous nilpotent Lie group $G$. We prove that operators $T$ of the form $T(f)= f*\mathcal K$ form an algebra under composition, and that such operators are bounded on $L^{p}(G)$ for $1<p<\infty$. 
Alexander Nagel, Fulvio Ricci, Elias M. Stein, Stephen Wainger
math.FA