Ordered group

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In abstract algebra, an ordered group is a group G equipped with a partial order "≤" which is translation-invariant; in other words, "≤" has the property that, for all a, b, and g in G, if ab then agbg and gagb. Note that sometimes the term ordered group is used for linearly/totally ordered group, and what we describe here is called partially ordered group.

By the definition we can reduce the partial order to a monadic property: ab iff 1a-1 b. The set of elements x1 of G are often denoted with G+. So we have ab iff a-1bG+. That G is an ordered group can be expressed only in terms of G+: A group is an ordered group iff there exists a subset H (which is G+) of G such that:

  • 1H
  • aH and bH then abH
  • if aH then x-1axH for each x of G

A typical example of an ordered group is Zn, where the group operation is componentwise addition, and we write (a1,...,an) ≤ (b1,...,bn) iff aibi (in the usual order of integers) for all i=1,...,n. More generally, if G is an ordered group and X is some set, then the set of all functions from X to G is again an ordered group: all operations are performed componentwise. Furthermore, every subgroup of G is an ordered group: it inherits the order from G.

If the order on the group is a linear order, we speak of a linearly ordered group.

If G and H are two ordered groups, a map from G to H is a morphism of ordered groups if it is both a group homomorphism and a monotonic function. The ordered groups, together with this notion of morphism, form a category.

Ordered groups are used in the definition of valuations of fields.