(Generalized) quasi-topological gravities at all orders

Abstract A new class of higher-curvature modifications of )-dimensional Einstein gravity has been recently identified. Densities belonging to this ‘Generalized quasi-topological’ class (GQTGs) are characterized by possessing non-hairy generalizations of the Schwarzschild black hole satisfying and by having second-order equations of motion when linearized around maximally symmetric backgrounds. GQTGs for which the equation of the metric function is algebraic are called ‘Quasi-topological’ and only exist for . In this paper we prove that GQTG and Quasi-topological densities exist in general dimensions and at arbitrarily high curvature orders. We present recursive formulas which allow for the systematic construction of n th order densities of both types from lower order ones, as well as explicit expressions valid at any order. We also obtain the equation satisfied by for general D and n . Our results here tie up the remaining loose end in the proof presented in Bueno et al (2019 (arXiv:1906.00987)) that every gravitational effective action constructed from arbitrary contractions of the metric and the Riemann tensor is equivalent, through a metric redefinition, to some GQTG.