@article{IM2_1969_3_6_a4,
     author = {N. S. Romanovskii},
     title = {Finite approximabiliyt of free products with respect to occurrence},
     journal = {Izvestiya. Mathematics },
     pages = {1245--1249},
     publisher = {mathdoc},
     volume = {3},
     number = {6},
     year = {1969},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1969_3_6_a4/}
}