# Directed set

In mathematics, a directed set (or a directed preorder or a filtered set) is a nonempty set A together with a reflexive and transitive binary relation ≤ (that is, a preorder), with the additional property that every pair of elements has an upper bound.[1] In other words, for any a and b in A there must exist c in A with ac and bc. A directed set's preorder is called a direction.

The notion defined above is sometimes called an upward directed set. A downward directed set is defined analogously,[2] meaning that every pair of elements is bounded below.[3] Some authors (and this article) assume that a directed set is directed upward, unless otherwise stated. Be aware that other authors call a set directed if and only if it is directed both upward and downward.[4]

Directed sets are a generalization of nonempty totally ordered sets. That is, all totally ordered sets are directed sets (contrast partially ordered sets, which need not be directed). Join semilattices (which are partially ordered sets) are directed sets as well, but not conversely. Likewise, lattices are directed sets both upward and downward.

In topology, directed sets are used to define nets, which generalize sequences and unite the various notions of limit used in analysis. Directed sets also give rise to direct limits in abstract algebra and (more generally) category theory.

## Equivalent definition

In addition to the definition above, there is an equivalent definition. A directed set is a set A with a preorder such that every finite subset of A has an upper bound. In this definition, the existence of an upper bound of the empty subset implies that A is nonempty.

## Examples

Examples of directed sets include:

• The set of natural numbers with the ordinary order is a directed set (and so is every totally ordered set).
• Let 𝔻1 and 𝔻2 be directed sets. Then the Cartesian product set 𝔻1 × 𝔻2 can be made into a directed set by defining (n1, n2) ≤ (m1, m2) if and only if n1m1 and n2m2. In analogy to the product order this is the product direction on the Cartesian product.
• It follows from previous example that the set ℕ × ℕ of pairs of natural numbers can be made into a directed set by defining (n0, n1) ≤ (m0, m1) if and only if n0m0 and n1m1.
• If T is a topological space and x0 is a point in T, set of all neighbourhoods of x0 can be turned into a directed set by writing UV if and only if U contains V. For every U, V, and W :
• UU since U contains itself.
• if UV and VW, then UV and VW, which implies UW. Thus UW.
• since x0UV, and since both UUV and VUV, we have UUV and VUV.
• Any preordered set (I, ≤) such that I has a greatest element is a directed set. An element gI is called a greatest element of (I, ≤) if jg for every jI.
• If (I, ≤) is a directed set and if mI is a maximal element of I then I necessarily has a unique greatest element, and that is m. An element mI is called a maximal element of (I, ≤) if there does not exist any jI such that jm and mj.
• If x0 is a real number then the set I := ℝ ∖ { x0 } can be turned into a directed set by defining aI b if |ax0| ≥ |bx0| (so "greater" elements are closer to x0). We then say that the reals have been directed towards x0. This is an example of a directed set that is neither partially ordered nor totally ordered. This is because antisymmetry breaks down for every pair a and b equidistant from x0, where a and b are on opposite sides of x0. Explicitly, this happens when { a, b } = { x0 - r, x0 + r } for some real r ≠ 0, in which case aI b and bI a even though ab. Had this preorder been defined on instead of ℝ ∖ { x0 } then it would still form a directed set but it would now have a (unique) greatest element, specifically x0; however, it still wouldn't be partially ordered. This example can be generalized to a metric space (X, d) by defining on X or X ∖ { x0 } the preorder ab if and only if d(a, x0) ≥ d(b, x0).
• A (trivial) example of a partially ordered set that is not directed is the set { a, b}, in which the only order relations are aa and bb. A less trivial example is like the previous example of the "reals directed towards x0" but in which the ordering rule only applies to pairs of elements on the same side of x0 (ie, if one takes an element a to the left of x0, and b to its right, then a and b are not comparable, and the subset {a, b} has no upper bound).
• A non-empty family of sets is a directed set with respect to the preorder (respectively, ) if and only if the intersection (resp., union) of any two of its members contains as a subset (resp., is contained as a subset of) some third member. In symbols, a family I of sets is directed with respect to (respectively, ) if and only if
for all A, BI, there exists some CI such that AC and BC (resp., AC and BC)
or equivalently,
for all A, BI, there exists some CI such that ABC (resp. ABC).
Every π-system, which is a non-empty family of sets that is closed under the intersection of any two of its members, is a directed set with respect to . Every λ-system is a directed set with respect to . Every filter, topology, and σ-algebra is a directed set with respect to both and .
• By definition a prefilter or filter base is a non-empty family of sets that is a directed set with respect to the partial order and that also doesn't contain the empty set (this condition prevents triviality because otherwise, the empty set would then be a greatest element with respect to ).
• In a poset P, every lower closure of an element, i.e. every subset of the form { a : aP, ax } where x is a fixed element from P, is directed.

## Contrast with semilattices

Example of a directed set which is not a join-semilattice

Directed sets are a more general concept than (join) semilattices: every join semilattice is a directed set, as the join or least upper bound of two elements is the desired c. The converse does not hold however, witness the directed set {1000,0001,1101,1011,1111} ordered bitwise (e.g. 1000 ≤ 1011 holds, but 0001 ≤ 1000 does not, since in the last bit 1 > 0), where {1000,0001} has three upper bounds but no least upper bound, cf. picture. (Also note that without 1111, the set is not directed.)

## Directed subsets

The order relation in a directed set is not required to be antisymmetric, and therefore directed sets are not always partial orders. However, the term directed set is also used frequently in the context of posets. In this setting, a subset A of a partially ordered set (P,≤) is called a directed subset if it is a directed set according to the same partial order: in other words, it is not the empty set, and every pair of elements has an upper bound. Here the order relation on the elements of A is inherited from P; for this reason, reflexivity and transitivity need not be required explicitly.

A directed subset of a poset is not required to be downward closed; a subset of a poset is directed if and only if its downward closure is an ideal. While the definition of a directed set is for an "upward-directed" set (every pair of elements has an upper bound), it is also possible to define a downward-directed set in which every pair of elements has a common lower bound. A subset of a poset is downward-directed if and only if its upper closure is a filter.

Directed subsets are used in domain theory, which studies directed-complete partial orders.[5] These are posets in which every upward-directed set is required to have a least upper bound. In this context, directed subsets again provide a generalization of convergent sequences.[further explanation needed]