Proof. Let one original graph be Eulerian and the other non-Eulerian but
connected. For necessity, suppose that the glued graph is Eulerian. Then by
Theorem 2.4, we obtain that the clone of the non-Eulerian graph contains all odd
vertices. Moreover, every even vertex in the clones of the two original graphs
is obtained from both even vertices in two original graphs. Conversely, suppose
that the clone of the non-Eulerian graph contains all odd vertices of the graph,
every even vertex in the clones of the two original graphs is obtained from both
even vertices in the two original graphs. Since odd vertices appear only in the
non-Eulerian graph, Theorem 2.4 concludes that the glued graph is Eulerian. ¥