@article{MZ_1970__117_171430,
     author = {Hanna Neumann and I.M.S. Dey},
     title = {The {Hopf} {Property} of {Free} {Products.}},
     journal = {Mathematische Zeitschrift},
     pages = {325--339},
     publisher = {mathdoc},
     volume = {117},
     year = {1970},
     zbl = {0204.34205},
     url = {http://geodesic.mathdoc.fr/item/MZ_1970__117_171430/}
}