@article{IM_1987__89_143473,
     author = {G. Harder},
     title = {Eisenstein cohomology of arithmetic groups. {The} case {GL2.}},
     journal = {Inventiones mathematicae},
     pages = {37--118},
     year = {1987},
     volume = {89},
     zbl = {0629.10023},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__89_143473/}
}