@article{IM2_2020_84_1_a3,
     author = {M. B. Karmanova},
     title = {Two-step {sub-Lorentzian} structures and graph surfaces},
     journal = {Izvestiya. Mathematics },
     pages = {52--94},
     publisher = {mathdoc},
     volume = {84},
     number = {1},
     year = {2020},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2020_84_1_a3/}
}