In the mathematical area of order theory, a completely distributive lattice is a complete lattice in which arbitrary joins distribute over arbitrary meets.

Formally, a complete lattice L is said to be completely distributive if, for any doubly indexed family {xj,k | j in J, k in Kj} of L, we have

${\displaystyle \bigwedge _{j\in J}\bigvee _{k\in K_{j))x_{j,k}=\bigvee _{f\in F}\bigwedge _{j\in J}x_{j,f(j)))$

where F is the set of choice functions f choosing for each index j of J some index f(j) in Kj.[1]

Complete distributivity is a self-dual property, i.e. dualizing the above statement yields the same class of complete lattices.[1]

Without the axiom of choice, no complete lattice with more than one element can ever satisfy the above property, as one can just let xj,k equal the top element of L for all indices j and k with all of the sets Kj being nonempty but having no choice function.[citation needed]

## Alternative characterizations

Various different characterizations exist. For example, the following is an equivalent law that avoids the use of choice functions[citation needed]. For any set S of sets, we define the set S# to be the set of all subsets X of the complete lattice that have non-empty intersection with all members of S. We then can define complete distributivity via the statement

{\displaystyle {\begin{aligned}\bigwedge \{\bigvee Y\mid Y\in S\}=\bigvee \{\bigwedge Z\mid Z\in S^{\#}\}\end{aligned))}

The operator ( )# might be called the crosscut operator. This version of complete distributivity only implies the original notion when admitting the Axiom of Choice.

## Properties

In addition, it is known that the following statements are equivalent for any complete lattice L:[2]

Direct products of [0,1], i.e. sets of all functions from some set X to [0,1] ordered pointwise, are also called cubes.

## Free completely distributive lattices

Every poset C can be completed in a completely distributive lattice.

A completely distributive lattice L is called the free completely distributive lattice over a poset C if and only if there is an order embedding ${\displaystyle \phi :C\rightarrow L}$ such that for every completely distributive lattice M and monotonic function ${\displaystyle f:C\rightarrow M}$, there is a unique complete homomorphism ${\displaystyle f_{\phi }^{*}:L\rightarrow M}$ satisfying ${\displaystyle f=f_{\phi }^{*}\circ \phi }$. For every poset C, the free completely distributive lattice over a poset C exists and is unique up to isomorphism.[3]

This is an instance of the concept of free object. Since a set X can be considered as a poset with the discrete order, the above result guarantees the existence of the free completely distributive lattice over the set X.

## Examples

• The unit interval [0,1], ordered in the natural way, is a completely distributive lattice.[4]
• The power set lattice ${\displaystyle ({\mathcal {P))(X),\subseteq )}$ for any set X is a completely distributive lattice.[1]
• For every poset C, there is a free completely distributive lattice over C.[3] See the section on Free completely distributive lattices above.