@article{KJM_2005_28_1_a4,
     author = {Johan Gielis and Stefan Haesen and Leopold Verstraelen},
     title = {Universal natural shapes},
     journal = {Kragujevac Journal of Mathematics},
     pages = {57 - 68},
     publisher = {mathdoc},
     volume = {28},
     number = {1},
     year = {2005},
     zbl = {1120.53300},
     url = {http://geodesic.mathdoc.fr/item/KJM_2005_28_1_a4/}
}