@article{MS2_1990__66_1_167094,
     author = {F. Malikow},
     title = {Singular vectors corresponding to imaginary roots in verma modules over affine {Lie} algebras.},
     journal = {Mathematica Scandinavica},
     pages = {73--90},
     publisher = {mathdoc},
     volume = {66},
     number = {1},
     year = {1990},
     zbl = {0687.17009},
     url = {http://geodesic.mathdoc.fr/item/MS2_1990__66_1_167094/}
}