@article{JRAM_1989__396_153122,
     author = {Bernhard Ruf and Th. Kappeler},
     title = {On the nodal line of the second eigenfunction of elliptic operators in two dimensions.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {1--13},
     publisher = {mathdoc},
     volume = {396},
     year = {1989},
     zbl = {0658.35068},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1989__396_153122/}
}