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.
Directed sets are a generalization of nonempty totally ordered sets, that is, all totally ordered sets are directed sets (but not all partially ordered sets). In topology, directed sets are used to...
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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.
Directed sets are a generalization of nonempty totally ordered sets, that is, all totally ordered sets are directed sets (but not all partially ordered sets). 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.
Examples of directed sets include:
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, where {1000,0001} has three...
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