@article{MS2_1990__67_2_167117,
     author = {G. Koester},
     title = {4-critical 4-valent planar graphs constructed with crowns.},
     journal = {Mathematica Scandinavica},
     pages = {15--22},
     publisher = {mathdoc},
     volume = {67},
     number = {2},
     year = {1990},
     zbl = {0723.05056},
     url = {http://geodesic.mathdoc.fr/item/MS2_1990__67_2_167117/}
}