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Encyclopedia > Dense order

In mathematics, a partial order ≤ on a set X is said to be dense (or dense-in-itself) if, for all x and y in X for which x < y, there is a z in X such that x < z < y. Euclid, Greek mathematician, 3rd century BC, known today as the father of geometry; shown here in a detail of The School of Athens by Raphael. ... In mathematics, a partially ordered set (or poset for short) is a set equipped with a special binary relation which formalizes the intuitive concept of an ordering. ...


The rational numbers with the ordinary ordering are a densely ordered set in this sense, as are the real numbers. On the other hand, the ordinary ordering on the integers is not dense. In mathematics, a rational number (commonly called a fraction) is a ratio or quotient of two integers, usually written as the vulgar fraction a/b, where b is not zero. ... In mathematics, the real numbers may be described informally in several different ways. ... The integers are commonly denoted by the above symbol. ...


See also


  Results from FactBites:
 
Math Forum Discussions - Re: which linearly ordered sets are sets of reals? (318 words)
> > A corollary is that having a countable dense subset
I didn't have in mind topological denseness necessarily.
The Math Forum is a research and educational enterprise of the Drexel School of Education.
Dense (198 words)
A partial order on a set S is said to be dense if, for all x and y in S for which x < y, there is a z in S such that x < z < y.
Note that the first notion of "dense" depends on the surrounding space, while the second notion is completely internal to the ordered set.
The rationals in [0,1] for instance are dense as an ordered set and they are dense in the space [0,1] but they are not dense in the real numbers.
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