A_n-Triality

A_n-type AGT correspondence anticipates that conformal blocks of A_n Toda CFT are related to partition functions of a family of 4d N=2 SCFTs. We use gauge/vortex duality to both give a precise form of the correspondence, and to prove it. Gauge/vortex duality relates the 4d theories and the 2d theories living on its vortices. Partition functions of the 2d theories on vortices provide Coulomb-gas representation of A_n Toda conformal blocks with discrete internal momenta. This gives a triality of relations between the gauge theory, its vortices and the Toda CFT. We prove that A_n triality holds for conformal blocks of A_n Toda on a sphere with all full punctures. The lift to one higher dimensional theories, compactified on a circle of arbitrary radius, and q-deformation of the Toda CFT, play a key role.