group theory

# Contents

## Definition

Let $G$ be a group. For the following this is often assumed to be (though is not necessarily) an abelian group. Hence we write here the group operation with a plus-sign

$+ : G \times G \to G \,.$

For $\mathbb{N}$ the natural numbers, there is a function

$(-)\cdot (-) : \mathbb{N} \times G \to G$

which takes a group element $g$ to

$n \cdot g \coloneqq \underbrace{g + g + \cdots + g}_{n \; summands} \,.$
###### Definition

A group $G$ is called divisible if for every natural number $n$ (hence for every integer) we have that for every element $g \in G$ there is an element $h \in G$ such that

$g = n \cdot h \,.$

In other words, if for every $n$ the ‘multiply by $n$’ map $G \stackrel{n}{\to} G$ is a surjection.

###### Definition

For $p$ a prime number a group is $p$-divisible if the above formula holds for all $n$ of the form $p^k$ for $k \in \mathbb{N}$.

###### Remark

There is also an abstract notion of $p$-divisible group in terms of group schemes.

## Properties

### Equivalent characterization

###### Proposition

Let $A$ be an abelian group.

Assuming the axiom of choice, the following are equivalent:

1. $A$ is divisible

2. $A$ is injective object in the the category Ab of abelian groups

3. the hom functor $Hom_{Ab}(-,A) : Ab^{op} \to Ab$ is exact.

This is for instance in (Tsit-YuenMoRi,Proposition 3.19). It follows for instance from using Baer's criterion.

### Stability under various operations

###### Proposition

The direct sum of divisible groups is itself divisible.

###### Proposition

Every quotient group of a divisible group is itself divisible.

## Examples

###### Example

The additive group of rational number $\mathbb{Q}$ is divisible. Hence also that underlying the real numbers $\mathbb{R}$ and the complex numbers.

Hence:

###### Example

The underlying abelian group of any $\mathbb{Q}$-vector space is divisible.

Also, by prop. 3,

###### Example

The quotient groups $\mathbb{Q}/\mathbb{Z}$ and $\mathbb{R}/\mathbb{Z}$ are divisible (the latter is also written $U(1)$ (for unitary group) or $S^1$ (for circle group)).

###### Remark

What is additionally interesting about example 3 is that it provides an injective cogenerator for the category Ab of abelian groups. Similarly, $\mathbb{R}/\mathbb{Z}$ is an injective cogenerator.

###### Counter-Example

The following groups are not divisible:

• the additive group of integers $\mathbb{Z}$.

• the cyclic group $\mathbb{Z}_n$ for $n \geq 1 \in \mathbb{N}$.

## References

• Tsit-Yuen Lam, Lectures on modules and rings, Graduate Texts in Mathematics No. 189, Berlin, New York (1999)
Revised on September 27, 2012 18:54:52 by Urs Schreiber (131.174.188.129)