Mathematical Journal of Okayama University (ISSN 0030-1566) was founded in 1952 by Mikao MORIYA, Takeshi INAGAKI, Masaru OSIMA and Tominosuke OTSUKI, and publishes original papers on pure and applied mathematics. Currently, each volume in principle appears annually and contains about 200 pages. All articles are peer reviewed by experts. Any communications relating to this publication should be addressed by email to : journal＠math.okayama-u.ac.jp or by post to : Editorial Office of Mathematical Journal of Okayama University, Department of Mathematics, Faculty of Science, Okayama University, Okayama 700-8530, Japan.