@misc{10_1017_S0017089500004274,
     title = {On the group ring of a free product with amalgamation},
     journal = {Glasgow mathematical journal},
     pages = {135--138},
     year = {1980},
     volume = {21},
     number = {2},
     doi = {10.1017/S0017089500004274},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500004274/}
}