pure subgroup
Definition. A pure subgroup H of an abelian group
G is
-
1.
a subgroup
of G, such that
-
2.
H∩mG=mH for all m∈ℤ.
The second condition says that for any h∈H such that h=ma for some integer m and some a∈G, then there exists b∈H such that h=mb. In other words, if h is divisible in G by an integer, then it is divisible in H by that same integer. Purity in abelian groups is a relative notion, and we denote H<pG to mean that H is a pure subgroup of G.
Examples. All groups mentioned below are abelian groups.
-
1.
For any group, two trivial examples of pure subgroups are the trivial subgroup and the group itself.
-
2.
Any divisible subgroup (http://planetmath.org/DivisibleGroup) or any direct summand
of a group is pure.
-
3.
The torsion subgroup (= the subgroup of all torsion elements) of any group is pure.
-
4.
If K<pH, H<pG, then K<pG.
-
5.
If H=⋃∞i=1Hi with Hi≤Hi+1 and Hi<pG, then H<pG.
-
6.
In Zn2, ⟨n⟩ is an example of a subgroup that is not pure.
-
7.
In general, ⟨m⟩<pZn if gcd(s,t)=1, where s=gcd(m,n) and t=n/s.
Remark. This definition can be generalized to modules over commutative rings.
Definition. Let R be a commutative ring and
ℰ:0→A→B→C→0 a short exact sequence of R-modules. Then
ℰ is said to be pure if it remains exact after
tensoring with any R-module. In other words, if D is any
R-module, then
D⊗ℰ:0→D⊗A→D⊗B→D⊗C→0, |
is exact.
Definition. Let N be a submodule of M over a ring R.
Then N is said to be a pure submodule of M if the exact
sequence
0→N→M→M/N→0 |
is a pure exact sequence.
From this definition, it is clear that H is a pure subgroup of G iff H is a pure ℤ-submodule of G.
Remark. N is a pure submodule of M over R iff whenever a finite sum
∑rimi=n∈N, |
where mi∈M and ri∈R implies that
n=∑rini |
for some ni∈N. As a result, if I is an ideal of R, then the purity of N in M means that N∩IM=IN, which is a generalization of the second condition in the definition of a pure subgroup above.
Title | pure subgroup |
---|---|
Canonical name | PureSubgroup |
Date of creation | 2013-03-22 14:57:47 |
Last modified on | 2013-03-22 14:57:47 |
Owner | CWoo (3771) |
Last modified by | CWoo (3771) |
Numerical id | 9 |
Author | CWoo (3771) |
Entry type | Definition |
Classification | msc 20K27 |
Classification | msc 13C13 |
Defines | pure submodule |
Defines | pure exact sequence |