RT Journal T1 Principal congruences on modal algebras A1 CAO Fa-sheng, XIAO Fang PB JOURNAL OF SHANDONG UNIVERSITY(NATURAL SCIENCE) FD YR 2020 JF JOURNAL OF SHANDONG UNIVERSITY(NATURAL SCIENCE) JO JOURNAL OF SHANDONG UNIVERSITY(NATURAL SCIENCE) VO 55 IS 2 SP 104 DO 10.6040/j.issn.1671-9352.0.2019.068 K1 principal congruence;principal congruence formula;Boolean algebra;modal algebra LK {http://lxbwk.njournal.sdu.edu.cn/EN/article/article_3232.shtml}