Goldblatt, Robert; Hodkinson, Ian Tangled closure algebras. (English) Zbl 1423.03258 Categ. Gen. Algebr. Struct. Appl. 7, No. 1, 9-31 (2017). Summary: The tangled closure of a collection of subsets of a topological space is the largest subset in which each member of the collection is dense. This operation models a logical ‘tangle modality’ connective, of significance in finite model theory. Here we study an abstract equational algebraic formulation of the operation which generalises the McKinsey-Tarski theory of closure algebras. We show that any dissectable tangled closure algebra, such as the algebra of subsets of any metric space without isolated points, contains copies of every finite tangled closure algebra. We then exhibit an example of a tangled closure algebra that cannot be embedded into any complete tangled closure algebra, so it has no MacNeille completion and no spatial representation. MSC: 03G25 Other algebras related to logic 06E25 Boolean algebras with additional operations (diagonalizable algebras, etc.) 06B23 Complete lattices, completions 54H10 Topological representations of algebraic systems Keywords:closure algebra; tangled closure; tangle modality; fixed point; quasi-order; Alexandroff topology; dense-in-itself; dissectable; MacNeille completion PDFBibTeX XMLCite \textit{R. Goldblatt} and \textit{I. Hodkinson}, Categ. Gen. Algebr. Struct. Appl. 7, No. 1, 9--31 (2017; Zbl 1423.03258) Full Text: arXiv