“…Clifford analysis may also be considered as a refinement of harmonic analysis, since, as does the Cauchy-Riemann operator in the complex plane, the rotation-invariant Dirac operator factorizes the Laplacian. The theory of Hardy spaces in Clifford analysis is by now well established, see [16,35,19,7], and the multidimensional Hilbert transform, as well as more general singular integral operators have been studied intensively, see [28,26,35,39,29,18,20], in particular on Lipschitz hypersurfaces, see [32,31,34], and on smooth closed hypersurfaces, such as the unit sphere, see [19,13].…”