@article{DCG_1987__2_131031,
     author = {D.Z. Du and F.K. Hwang and G.D. Song and G.Y Ting},
     title = {Steiner {Minimal} {Trees} on {Sets} of {Four} {Points.}},
     journal = {Discrete & computational geometry},
     pages = {401--414},
     publisher = {mathdoc},
     volume = {2},
     year = {1987},
     zbl = {0623.05013},
     url = {http://geodesic.mathdoc.fr/item/DCG_1987__2_131031/}
}