Conformal invariant powers of the Laplacian, Fefferman-Graham ambient metric and Ricci gauging

The hierarchy of conformally invariant kth powers of the Laplacian acting on a scalar field with scaling dimensions Δ(k)=k−d/2, k=1,2,3, as obtained in the recent work [R. Manvelyan, D.H. Tchrakian, Phys. Lett. B 644 (2007) 370, hep-th/0611077] is rederived using the Fefferman-Graham (d+2)-dimensional ambient space approach. The corresponding mysterious “holographic” structure of these operators is clarified. We explore also the (d+2)-dimensional ambient space origin of the Ricci gauging procedure proposed by A. Iorio, L. O'Raifeartaigh, I. Sachs and C. Wiesendanger as another method of constructing the Weyl invariant Lagrangians. The corresponding gauged ambient metric, Fefferman-Graham expansion and extended Penrose-Brown-Henneaux transformations are proposed and analyzed.