*** 2021/11/18（Thu）*** The volume 64 (January, 2022) is now available online. *** 2021/04/14（Wed）*** We encourage author(s) to submit his/her/their paper through the submission page. *** 2021/01/01（Fri）*** We invite you to submit an article to Mathematical Journal of Okayama University (MJOU) which is a peer-reviewed, open access journal that publishes original research articles in pure and applied mathematics. We publish original research papers rapidly and free of charge. The corresponding author will receive 40 hardcopy reprints free of charge, this number to be shared between joint authors. *** 2020/12/25（Fri）*** MJOU started to assign DOI to each paper.