@article{IM2_2008_72_4_a0,
     author = {A. V. Alekseevskii and S. M. Natanzon},
     title = {The algebra of bipartite graphs and {Hurwitz} numbers of seamed surfaces},
     journal = {Izvestiya. Mathematics },
     pages = {627--646},
     publisher = {mathdoc},
     volume = {72},
     number = {4},
     year = {2008},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2008_72_4_a0/}
}