@article{MAN_1994__300_2_165239,
     author = {Oldrich Kowalski and Friedbert Pr\"ufer},
     title = {On {Riemannian} 3-manifolds with distinct constant {Ricci} eigenvalues.},
     journal = {Mathematische Annalen},
     pages = {17--28},
     publisher = {mathdoc},
     volume = {300},
     number = {2},
     year = {1994},
     zbl = {0813.53020},
     url = {http://geodesic.mathdoc.fr/item/MAN_1994__300_2_165239/}
}