1. Gary Chartrand:
Department of Mathematics, Western Michigan University,Kalamazoo, Michigan ,USA
2. James Hallas:
Department of Mathematics, Western Michigan University,Kalamazoo, Michigan ,USA
3. Ebrahim Salehi:
Department of Mathematical Sciences, University of Nevada Las Vegas,Las Vegas, Nevada 89154-4020,USA
4. Ping Zhang:
Department of Mathematics, Western Michigan University,Kalamazoo, Michigan 49008-5248,USA