# Pseudocomplement

In mathematics, particularly in order theory, a pseudocomplement is one generalization of the notion of complement. In a lattice L with bottom element 0, an element xL is said to have a pseudocomplement if there exists a greatest element x* ∈ L, disjoint from x, with the property that xx* = 0. More formally, x* = max{ yL | xy = 0 }. The lattice L itself is called a pseudocomplemented lattice if every element of L is pseudocomplemented. Every pseudocomplemented lattice is necessarily bounded, i.e. it has a 1 as well. Since the pseudocomplement is unique by definition (if it exists), a pseudocomplemented lattice can be endowed with a unary operation * mapping every element to its pseudocomplement; this structure is sometimes called a p-algebra. However this latter term may have other meanings in other areas of mathematics.

## Properties

In a p-algebra L, for all x, yL:

• The map xx* is antitone. In particular, 0* = 1 and 1* = 0.
• The map xx** is a closure.
• x* = x***.
• (xy)* = x* ∧ y*.
• (xy)** = x** ∧ y**.

The set S(L) ≝ { x** | xL } is called the skeleton of L. S(L) is a ∧-subsemilattice of L and together with xy = (xy)** = (x* ∧ y*)* forms a Boolean algebra (the complement in this algebra is *). In general, S(L) is not a sublattice of L. In a distributive p-algebra, S(L) is the set of complemented elements of L.

Every element x with the property x* = 0 (or equivalently, x** = 1) is called dense. Every element of the form xx* is dense. D(L), the set of all the dense elements in L is a filter of L. A distributive p-algebra is Boolean if and only if D(L) = {1}.

Pseudocomplemented lattices form a variety.

## Examples

• Every finite distributive lattice is pseudocomplemented.
• Every Stone algebra is pseudocomplemented. In fact, a Stone algebra can be defined as a pseudocomplemented distributive lattice L in which any of the following equivalent statements hold for all x, yL:
• S(L) is a sublattice of L;
• (xy)* = x* ∨ y*;
• (xy)** = x** ∨ y**;
• x* ∨ x** = 1.
• Every Heyting algebra is pseudocomplemented.
• If X is a set, the open set topology on X is a pseudocomplemented (and distributive) lattice with the meet and join being the usual union and intersection of open sets. The pseudocomplement of an open set A is the interior of the set complement of A. Furthermore, the dense elements of this lattice are exactly the dense open subsets in the topological sense.

## Relative pseudocomplement

A relative pseudocomplement of a with respect to b is a maximal element c such that acb. This binary operation is denoted ab. A lattice with the pseudocomplement for each two elements is called implicative lattice, or Brouwerian lattice. In general case, an implicative lattice may not have a minimal element, if such element exists, then pseudocomplement a* could be defined using relative pseudocomplement as a → 0.

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