corresponding to the field. (Historically the term complex was first used in the case where the algebraic structure was a group and has its origins in 19th century group theory where a subset of a group was called a complex.) See also Notes References Goldblatt, R., Algebraic Polymodal Logic: A Survey, Logic Journal of the IGPL, Volume 8, Issue 4, p. 393-450, July 2000 Goldblatt, R., Varieties of complex algebras, Annals of Pure and Applied Logic, 44, p. 173-242, 1989 Naturman, C.A., Interior Algebras and Topology, Ph.D. thesis, University of Cape Town Department of Mathematics, 1991 Patrick Blackburn, Johan F.A.K. van Benthem, Frank Wolter ed., Handbook of Modal Logic, Volume 3 of Studies in Logic and Practical Reasoning, Elsevier, 2006 External links Algebra of sets, Encyclopedia of Mathematics. Boolean algebra Families of sets