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A geometric derivation is given of a family of scalar conformally invariant differential operators on conformal densities with leading part a power of the Laplacian. The derivation produces an operator for each positive integral power in odd dimensions, but only for a finite range of powers in even dimensions. The operators are derived from the Laplacian in the ambient metric of Fefferman and Graham via extension.
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Graham et al. (1992) studied this question.
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