ABSTRACT
Nowhere-zero flows on graphswere introduced by Tutte [30-32] as a dual concept to graph coloring problems. A graph has a nowhere-zero k-flow if its edges can be oriented and assigned numbers ±1, …, ±(k − 1), so that for each vertex, the sum value on the incoming edges equals the sum on the outgoing ones. By Tutte, a planar
FIGURE 5.1: Three different drawings of the Petersen graph.
