Unitary Truncations and Critical Gravity: a Toy Model
  We investigate a higher-derivative scalar field model in a fixed d+1 dimensional AdS background as a toy model for a gravitational dual to a higher-rank logarithmic CFT. The holographic two-point correlation functions on the boundary agree with higher-rank LCFT correlation functions. For odd rank, the theory allows for a truncation to a nontrivial subspace with non-negative scalar product. We discuss possible implications for higher-derivative critical gravity theories. 
Eric A. Bergshoeff, Sjoerd de Haan, Wout Merbis, Massimo Porrati and   Jan Rosseel
hep-th