Figure 7.

An example for the proof of Theorem 2. Part (a) shows a non-trivial connected component C of a trajectory graph. Part (b) is the corresponding adjacency graph A = {I(C), O(C), E}, which has two cycles. All extremities in the first cycle are shown bold. The superscripts 1, 2 and 3 on each adjacency represent labels of a₁, a₂ and a₃ respectively. After removing all adjacency nodes in (a) labeled as a₁ or a₂, the remaining part is shown in part (c), in which all nodes have even total degree.