1. Faculty of Engineering, Kinki University 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
2. Professor emeritus, Kinki University 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
3. Department of Mathematics, Kinki University 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan