1. Missouri University of Science and Technology, Department of Economics and Finance, 103 Harris Hall, Rolla, MO 65409-1250, USA
2. Mathematical Institute of the Serbian Academy of Science Knez Mihailova 36/III, 11000 Beograd, Serbia
3. Missouri University of Science and Technology, Department of Mathematics and Statistics, 314 Rolla Building, Rolla, MO 65409-0020, USA