@article{MS2_1977__40_166511,
     author = {Fr. Fabricius-Bjerre},
     title = {A relation between the numbers of singular points and singular lines of a plane closed curve.},
     journal = {Mathematica Scandinavica},
     pages = {20--24},
     publisher = {mathdoc},
     volume = {40},
     year = {1977},
     zbl = {0352.50011},
     url = {http://geodesic.mathdoc.fr/item/MS2_1977__40_166511/}
}