@article{RM_1990_45_1_a20,
     author = {S. L. Tabachnikov},
     title = {Around four vertices},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {229--230},
     publisher = {mathdoc},
     volume = {45},
     number = {1},
     year = {1990},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1990_45_1_a20/}
}