@article{MT_2008_11_2_a3,
     author = {A. G. Kremlyov and Yu. G. Nikonorov},
     title = {The signature of the {Ricci} curvature of left-invariant {Riemannian} metrics on four-dimensional {Lie} groups. {The} unimodular case},
     journal = {Matemati\v{c}eskie trudy},
     pages = {115--147},
     publisher = {mathdoc},
     volume = {11},
     number = {2},
     year = {2008},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MT_2008_11_2_a3/}
}