“…The class K. The purpose of this subsection is to give some examples of operators in the class K. More precisely, the class K contains almost all important operators in Harmonic analysis: Calderón-Zygmund type operators, strongly singular integral operators, multiplier operators, pseudo-differential operators with symbols in the Hörmander class S m ̺,δ with 0 < ̺ ≤ 1, 0 ≤ δ < 1, m ≤ −n((1 − ̺)/2 + max{0, (δ − ̺)/2}) (see [2,1]), maximal type operators, the area integral operator of Lusin, Littlewood-Paley type operators, Marcinkiewicz operators, maximal Bochner-Riesz operators T δ * with δ > (n − 1)/2 (cf. [24]), etc...…”