@article{SM_1976_29_2_a9,
     author = {G. S. Makanin},
     title = {On the rank of coefficientless equations in four unknowns in a~free semigroup},
     journal = {Sbornik. Mathematics},
     pages = {257--280},
     publisher = {mathdoc},
     volume = {29},
     number = {2},
     year = {1976},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1976_29_2_a9/}
}