@article{10_1017_S0017089500007849,
     author = {Hannebauer, Torsten},
     title = {Relation modules of amalgamated free products and {HNN} extensions},
     journal = {Glasgow mathematical journal},
     pages = {263--270},
     year = {1989},
     volume = {31},
     number = {3},
     doi = {10.1017/S0017089500007849},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500007849/}
}