Affiliation: | a Department of Mathematics, Wakayama National College of Technology, Nojima 77, Nada-machi, Gobo, Wakayama 644-0023, Japan b Graduate School of Advanced Technology, Kinki University, Iizuka, Fukuoka 820-8555, Japan c Graduate School of Mathematics, Nagoya University, Nagoya 464-8602, Japan d Department of Mathematics, Northwest University, Xi'an, Shaanxi, People's Republic of China |