Fundamental solutions of a class of ultra-hyperbolic operators on pseudo H-type groups

Abstract

Pseudo H-type Lie groups Gr,s of signature (r,s) are defined via a module action of the Clifford algebra Cℓr,s on a vector space V≅R2n. They form a subclass of all 2-step nilpotent Lie groups. Based on their algebraic structure they can be equipped with a left-invariant pseudo-Riemannian metric. Let Nr,s denote the Lie algebra corresponding to Gr,s. In the case s>0 a choice of left-invariant vector fields [X1,…,X2n] which generate a complement of the center of Nr,s gives rise to a second order differential operatorΔr,s:=(X12+…+Xn2)−(Xn+12+…+X2n2), which we call ultra-hyperbolic. We prove that Δr,s is locally solvable if and only if r=0. In particular, it follows that Δr,s does not admit a fundamental solution in the space D′(Gr,s) of Schwartz distributions whenever r>0. In terms of classical special functions we present families of fundamental solutions of Δ0,s in the class of tempered distributions S′(G0,s) and study their properties.