(of a set in a topological space:)

A set is dense iff its closure is the entire space.

In a metric space, this reduces to the following: for every (positive) ε and every point x in the space, there exists a point y in the set such that d(x,y) < ε.

For instance, the set of rationals is dense inside the set of real numbers. Another example: A theorem of Weierstrass is that the set of polynomials is dense inside C([0,1]) (the set of continuous functions on [0,1], with the "L-infinity" topology of uniform convergence.

(of an order type or a total order:)

An ordered set (A,<) is dense iff there is an element of A between any two elements of A: ∀x,y∈A: (x<y) → ∃z∈A: x<z&z<y.

Note that only a single set participates in this definition of set -- not 2, like in the topological definition above!

The concept may also be extended further to a partially ordered set (a poset) by using the same formula.

For instance, the set of rational numbers with their regular ordering is dense: for any x,y, we can take (x+y)/2 (or even (2x+3y)/5), which lies between them.

Dense (?), a. [L. densus; akin to Gr. thick with hair or leaves: cf. F. dense.]

1.

Having the constituent parts massed or crowded together; close; compact; thick; containing much matter in a small space; heavy; opaque; as, a dense crowd; a dense forest; a dense fog.

All sorts of bodies, firm and fluid, dense and rare. Ray.

To replace the cloudy barrier dense. Cowper.

2.

Stupid; gross; crass; as, dense ignorance.

 

© Webster 1913.

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