Trudeau, Richard J. (1993). Introduction to Graph Theory (ed. Corrected, enlarged republication.). New York: Dover Pub.. pp. 64. ISBN 978-0-486-67870-2. http://store.doverpublications.com/0486678709.html. Accesat la 8 august 2012. „Thus a planar graph, when drawn on a flat surface, either has no edge-crossings or can be redrawn without them.”
