Peng Ren,Zhiyuan Tian,Zeyu Wang,Xin Li,Xia Wang,Tao Zhao,Ming ShengIn this Chapter we consider operators of the form A = F.Ф(ξ)F., where F is the Fourier transform on ., and Ф is a positively homogeneous function of complex degree ., i.e. Ф(.) = .Ф(. for . ∈.