@article{ZNSL_1985_143_a7,
     author = {S. V. Matveev},
     title = {Structure of the second homotopy group of the union of two spaces},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {147--155},
     publisher = {mathdoc},
     volume = {143},
     year = {1985},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1985_143_a7/}
}