The Verlinde formula in logarithmic CFT

In rational conformal field theory, the Verlinde formula computes the fusion coefficients from the modular S-transformations of the characters of the chiral algebra's representations. Generalising this formula to logarithmic models has proven rather difficult for a variety of reasons. Here, a recently proposed formalism [1] for the modular properties of certain classes of logarithmic theories is reviewed, and refined, using simple examples. A formalism addressing fusion rules in simple current extensions is also reviewed as a means to tackle logarithmic theories to which the proposed modular formalism does not directly apply.DR’s research is supported by the Australian Research Council Discovery Project DP1093910. SW’s work is supported by the Australian Research Council Discovery Early Career Researcher Award DE140101825