Zdenek Dvorak Riste Skrekovski T. Valla Planar Graphs of Odd-Girth at Least 9 are Homomorphic to the Petersen Graph. 568-591 2008 22 SIAM J. Discrete Math. 2 http://dx.doi.org/10.1137/060650507 db/journals/siamdm/siamdm22.html#DvorakSV08