@article{IM_1986__83_143309,
     author = {Reese Harvey and Frank Morgan},
     title = {The faces of the {Grassmanian} of three-planes in {R7} {(Calibrated} geometries on {R7).}},
     journal = {Inventiones mathematicae},
     pages = {191--228},
     publisher = {mathdoc},
     volume = {83},
     year = {1986},
     zbl = {0556.53036},
     url = {http://geodesic.mathdoc.fr/item/IM_1986__83_143309/}
}