Guram Bezhanishvili Silvio Ghilardi Mamuka Jibladze An Algebraic Approach to Subframe Logics. Modal Case. 187-202 2011 52 Notre Dame Journal of Formal Logic 2 http://projecteuclid.org/euclid.ndjfl/1303995714 db/journals/ndjfl/ndjfl52.html#BezhanishviliGJ11