有序群/有序交换群

lilianjie

In abstract algebra, a partially ordered group is a group (G,+) equipped with a partial order "≤" that is translation-invariant; in other words, "≤" has the property that, for all a, b, and g in G, if a ≤ b then a+g ≤ b+g and g+a ≤ g+b.

An element x of G is called positive element if 0 ≤ x. The set of elements 0 ≤ x is often denoted with G+, and it is called the positive cone of G. So we have a ≤ b if and only if -a+b ∈ G+.
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* v6 v4 S8 v" _% q  iBy the definition, we can reduce the partial order to a monadic property: a ≤ b if and only if 0 ≤ -a+b.

# ?/ `, B+ Y3 X% s6 n) @+ o# cFor the general group G, the existence of a positive cone specifies an order on G. A group G is a partially ordered group if and only if there exists a subset H (which is G+) of G such that:

7 u8 }5 f/ ?! ~3 C# S6 S! o; E0 ∈ H
8 x; _6 W# g& ?  X( Mif a ∈ H and b ∈ H then a+b ∈ H
if a ∈ H then -x+a+x ∈ H for each x of G
if a ∈ H and -a ∈ H then a=0
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lilianjie

有序交换群系指一对 (Γ, > )，其中 Γ 为交换群， > 为其上的一个二元关系，且满足如下条件：

1 Q3 o- B$ i7 g9 u! u2 k若 a < 0，则 − a > 0。
. j$ ?+ f9 k& V5 G4 z  ?2 S! D若 a,b > 0，则 a + b > 0。

lilianjie

Examples
An ordered vector space is a partially ordered group
* n  L" d. h9 I4 c# sA Riesz space is a lattice-ordered group
1 V" {8 m. o7 u0 Q# _9 z5 AA typical example of a partially ordered group is Zn, where the group operation is componentwise addition, and we write (a1,...,an) ≤ (b1,...,bn) if and only if ai ≤ bi (in the usual order of integers) for all i=1,...,n.
: @% [$ K" V4 n( b9 TMore generally, if G is a partially ordered group and X is some set, then the set of all functions from X to G is again a partially ordered group: all operations are performed componentwise. Furthermore, every subgroup of G is a partially ordered group: it inherits the order from G.
6 k( K9 ~. i; t4 E2 Y& R/ U序线性空间是有序群
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Z/R/R*都是有序交换群

孤寂冷逍遥