# Totally ordered set

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chain

A partially ordered set in which for any two elements and either or . A subset of a totally ordered set is itself a totally ordered set. Every maximal (minimal) element of a totally ordered set is a largest (smallest) element. An important special case of totally ordered sets are the well-ordered sets (cf. Well-ordered set). Among the subsets of a partially ordered set that are totally ordered sets, a particularly important role is played by a composition sequence. A cut of a totally ordered set is a partition of it into two subsets and such that , is empty, and , where

The classes and are called the lower and upper classes of the cut. One can distinguish the following types of cuts: a jump — there is a largest element in the lower class and a smallest element in the upper class; a Dedekind cut — there is a largest (smallest) element in the lower (upper) class, but no smallest (largest) element in the upper (lower) class; a gap — there is no largest element in the lower class and no smallest element in the upper class. A totally ordered set is said to be continuous if all its cuts are Dedekind cuts. A subset of a totally ordered set is said to be dense if every interval of not reducing to a single element contains elements belonging to . The totally ordered set of real numbers can be characterized as a continuous totally ordered set which has neither a largest nor a smallest element, but which contains a countable dense subset. Every countable totally ordered set is isomorphic to some subset of the totally ordered set of all binary fractions in the interval . A lattice is isomorphic to a subset of the totally ordered set of integers if and only if every sublattice of it is a retract.

#### References

 [1] P.S. Aleksandrov, "Einführung in die Mengenlehre und die Theorie der reellen Funktionen" , Deutsch. Verlag Wissenschaft. (1956) (Translated from Russian) [2] P.S. Aleksandrov, "Einführung in die Mengenlehre und in die allgemeine Topologie" , Deutsch. Verlag Wissenschaft. (1984) (Translated from Russian) [3] N. Bourbaki, "Elements of mathematics. Theory of sets" , Addison-Wesley (1968) (Translated from French)