1Department of Integrated Design Engineering, Maebashi Institute of Technology, Maebashi 371-0816 Japan
2Department of Mathematics and Computer Science, Drew University, Madison, NJ 07940 USA
3 Department of Mathematics, Western Michigan University, Kalamazoo, MI 49008 USA
4Department of Computer Science and Information Theory, Budapest Univer- sity of Technology and Economics, 1521 Budapest, P.O. Box 91 Hungary,