Negative Generators of the Virasoro Constraints for the BKP Hierarchy
  We give a straightforward derivation of the string equation and Virasoro constraints on the $\tau$ function of the BKP hierarchy by means of some special additional symmetry flows. The explicit forms of the actions of these additional symmetry flows on the wave function and then the negative Virasoro generators $L_{-k}$ are given, where $k$ is a positive integer. 
Jingsong He, Kelei Tian, Angela Foerster