@article{MS2_1976__39_166500,
     author = {Helge Tverberg},
     title = {On a four-colour theorem.},
     journal = {Mathematica Scandinavica},
     pages = {267--270},
     publisher = {mathdoc},
     volume = {39},
     year = {1976},
     zbl = {0347.05104},
     url = {http://geodesic.mathdoc.fr/item/MS2_1976__39_166500/}
}