1. Department of Industrial and Systems Engineering, Hiroshima University, 4-1 Kagamiyama 1 Chome, Higashi-Hiroshima 739, Japan
2. Department of Economic Sciences, Hiroshima Shudo University, 1717 Ohtsuka, Numata-cho, Asaminami-ku, Hiroshima 731-31, Japan
3. Department of Industrial and Systems Engineering, Hiroshima University, 4-1 Kagamiyama 1 Chome, Higashi-Hiroshima 739, Japan