Textbook: Introduction to Graph Theory by Robert J. Wilson 5th Edition Pg. 86

4.10 A planar graph G is outerplanar if G can be drawn in the plane so that all of its ver- tices lie on the exterior boundary. (i) Show that K, and K23 are not outerplanar. (ii) Deduce that, if G is an outerplanar graph, then G contains no subgraph homeo- morphic or contractible to K, or K2.3- (The converse result also holds, yielding a Kuratowski-type criterion for a graph to be outerplanar.)