The Laplace operator on Rn satisfies the equation ∆(fg)(x) = (∆f)(x)g(x) + f(x)(∆g)(x) + 2 〈f ′(x), g′(x)〉 for all f, g ∈ C2(Rn,R) and x ∈ Rn. We consider an operator equation generalizing this product formula. Suppose T : C2(Rn,R) → C(Rn,R) and A : C2(Rn,R) → C(Rn,Rn) are operators satisfying the equation T (fg)(x) = (Tf)(x)g(x) + f(x)(Tg)(x) + 〈(Af)(x), (Ag)(x)〉 (1) for all f, g ∈ C2(Rn,R) and x ∈ Rn . Assume, in addition, that T is O(n)-invariant, annihilates the affine functions and that A is non-degenerate. Then T is a multiple of the Laplacian on Rn , and A a multiple of the derivative, (Tf)(x) = d(||x||)2 2 (∆f)(x) , (Af)(x) = d(||x||)f ′(x) where d ∈ C(R+,R) is a continuous function. We describe the solutions as well, if T is not O(n)-invariant or does not annihilate the affine functions. To do so, we determine all operators (T,A) satisfying (1) for scalar operators A : C2(Rn,R) → C(Rn,R) . The map A, both in the vector and the scalar case, is closely related to T and there are just three different types of solution operators (T,A) . No continuity or linearity requirement is imposed on T or A . 1 Statement of the main results. It recently became clear that some fundamental operations in analysis and geometry like derivatives or duality maps are characterized by simple properties or operator functional equations: the derivative is characterized by the chain or the Leibniz rule, cf. [AAM], [AKM], [KM1], [KM2], the duality of convex bodies and the Legendre transform as a ∗Supported in part by the Alexander von Humboldt Foundation, by ISF grant 387/09 and BSF grant 2006079.
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