Dense order

Uniqueness for total dense orders without endpoints

Georg Cantor proved that every two non-empty dense totally ordered countable sets without lower or upper bounds are order-isomorphic. This makes the theory of dense linear orders without bounds an example of an ω-categorical theory where ω is the smallest limit ordinal. For example, there exists an order-isomorphism between the rational numbers and other densely ordered countable sets including the dyadic rationals and the algebraic numbers. The proofs of these results use the back-and-forth method.

Minkowski's question mark function can be used to determine the order isomorphisms between the quadratic algebraic numbers and the rational numbers, and between the rationals and the dyadic rationals.

Generalizations

Any binary relation R is said to be dense if, for all R-related x and y, there is a z such that x and z and also z and y are R-related. Formally:

${\displaystyle \mathrm{\forall }x\mathrm{\forall }yxRy\Rightarrow (\mathrm{\exists }zxRz\wedge zRy).}$ Alternatively, in terms of composition of R with itself, the dense condition may be expressed as R ⊆ (R ; R).

Sufficient conditions for a binary relation R on a set X to be dense are:

• R is reflexive;
• R is coreflexive;
• R is quasireflexive;
• R is left or right Euclidean; or
• R is symmetric and semi-connex and X has at least 3 elements.

None of them are necessary. For instance, there is a relation R that is not reflexive but dense. A non-empty and dense relation cannot be antitransitive.

A strict partial order < is a dense order if and only if < is a dense relation. A dense relation that is also transitive is said to be idempotent.