Fefferman-Graham ambient metrics of Patterson-Walker metrics
  Given an $n$-dimensional manifold $N$ with an affine connection $D$, we show that the associated Patterson-Walker metric $g$ on $T^*N$ admits a global and explicit Fefferman-Graham ambient metric. This provides a new and large class of conformal structures which are generically not conformally Einstein but for which the ambient metric exists to all orders and can be realized in a natural and explicit way. In particular, it follows that Patterson-Walker metrics have vanishing Fefferman-Graham obstruction tensors. As an application of the concrete ambient metric realization we show in addition that Patterson-Walker metrics have vanishing Q-curvature. 
Matthias Hammerl, Katja Sagerschnig, Josef \v{S}ilhan, Arman   Taghavi-Chabert, Vojt\v{e}ch \v{Z}\'adn\'ik
math.DG