# Complete lattice

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A partially ordered set in which any subset has a least upper bound and a greatest lower bound. These are usually called the join and the meet of and are denoted by and or simply by and (respectively). If a partially ordered set has a largest element and each non-empty subset of it has a greatest lower bound, then it is a complete lattice. A lattice is complete if and only if any isotone mapping of the lattice into itself has a fixed point, i.e. an element such that . If is the set of subsets of a set ordered by inclusion and is a closure operation on , then the set of all -closed subsets is a complete lattice. Any partially ordered set can be isomorphically imbedded in a complete lattice, which in that case is called a completion of . The completion by sections (cf. Completion, MacNeille (of a partially ordered set)) is the least of all completions of a given partially ordered set. Complete lattices are formed by the set of all subalgebras in a universal algebra, by the set of all congruences in a universal algebra, and by the set of all closed subsets in a topological space.

#### References

 [1] G. Birkhoff, "Lattice theory" , Colloq. Publ. , 25 , Amer. Math. Soc. (1973) [2] L.A. Skornyakov, "Elements of lattice theory" , Hindushtan Publ. Comp. (1977) (Translated from Russian)