1. A. Kaplan, “Fundamental solutions for a class of hypoelliptic PDE generated by composition of quadratics forms,” Trans. Amer.Math. Soc. 258 (1), 147–153 (1980).
2. A. Bonfiglioli, E. Lanconelli, and F. Uguzzoni, Stratified Lie Groups and Potential Theory for Their Sub-Laplacians (Springer, Berlin, 2007).
3. E. M. Stein, Harmonic Analysis. Real-Variable Methods, Orthogonality and Oscillatory Integrals, in Princeton Math. Ser. (Princeton Univ. Press, Princeton, NJ, 1993), Vol. 43.
4. G. B. Folland and E. M. Stein, Hardy Spaces on Homogeneous Groups, in Math. Notes (Princeton Univ. Press, Princeton, 1982), Vol. 28.
5. G. B. Folland and E.M. Stein, “Estimates for the ∂b-complex and analysis on the Heisenberg group,” Comm. Pure Appl.Math. 27, 429–522 (1974).