@article{IM2_1993_40_1_a0,
     author = {D. V. Alekseevskii and M. M. Graev},
     title = {Twistors and $G$-structures},
     journal = {Izvestiya. Mathematics},
     pages = {1--31},
     year = {1993},
     volume = {40},
     number = {1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1993_40_1_a0/}
}