@article{701394,
     title = {Unitary representations of the Virasoro algebra},
     booktitle = {Proceedings of the Winter School "Geometry and Physics"},
     series = {GDML\_Books},
     publisher = {Circolo Matematico di Palermo},
     address = {Palermo},
     year = {1985},
     pages = {[129]-135},
     mrnumber = {MR0853135},
     zbl = {0593.17012},
     url = {http://dml.mathdoc.fr/item/701394}
}

Kent, Adrian. Unitary representations of the Virasoro algebra, dans Proceedings of the Winter School "Geometry and Physics", GDML_Books,  (1985), pp. [129]-135. http://gdmltest.u-ga.fr/item/701394/