A note on the Virasoro blocks at order 1 / c.

We derive an explicit expression for the 1 / c contribution to the Virasoro blocks in 2D CFT in the limit of large c with fixed values of the operators' dimensions. We follow the direct approach of orthonormalising, at order 1 / c, the space of the Virasoro descendants to obtain the blocks as a series expansion around z = 0 . For generic conformal weights this expansion can be summed in terms of hypergeometric functions and their first derivatives with respect to one parameter. For integer conformal weights we provide an equivalent expression written in terms of a finite sum of undifferentiated hypergeometric functions. These results make the monodromies of the blocks around z = 1 manifest.

Unitary 4-point correlators from classical geometries.

We compute correlators of two heavy and two light operators in the strong coupling and large c limit of the D1D5 CFT which is dual to weakly coupled AdS 3 gravity. The light operators have dimension two and are scalar descendants of the chiral primaries considered in arXiv:1705.09250, while the heavy operators belong to an ensemble of Ramond-Ramond ground states. We derive a general expression for these correlators when the heavy states in the ensemble are close to the maximally spinning ground state. For a particular family of heavy states we also provide a result valid for any value of the spin. In all cases we find that the correlators depend non-trivially on the CFT moduli and are not determined by the symmetries of the theory; however, they have the properties expected for correlators among pure states in a unitary theory, in particular they do not decay at large Lorentzian times.