# Homework Help: Help with Direct Sums of Groups

1. Sep 27, 2012

### thoughtinknot

1. The problem statement, all variables and given/known data
Let $\mathbb{R}$*=$\mathbb{R}$\{0} with multiplication operation. Show that $\mathbb{R}$*=$\mathbb{I}$2 ⊕ $\mathbb{R}$, where the group operation in $\mathbb{R}$ is addition.

2. Relevant equations
Let {A1,...,An}$\subseteq$A such that for all a$\in$A there exists a unique sequence {ak} such that a=a1+...+an where ak$\in$Ak for all k, then A=A1⊕...⊕An

3. The attempt at a solution
Since $\mathbb{I}$2={-1,1} I don't think I can show that every a*$\in$$\mathbb{R}$* can be expressed in a unique way. For example let a+=a*+1 and a-=a*-1, then a*=a+-1=a-+1. Am I defining the cyclic group of order 2 wrong? I'm not that sure about direct sums, our prof spent 5 minutes on them and 40% of our assignment involves them :S

Last edited: Sep 27, 2012
2. Sep 28, 2012

### thoughtinknot

Am I wrong in thinking this question is incorrect since $\mathbb{R}$ is not contained in $\mathbb{R}$*, thus $\mathbb{R}$* ≠ $\mathbb{I}$2 ⊕ $\mathbb{R}$?

3. Sep 28, 2012

### jgens

The question is correct. Consider the exponential map.

4. Sep 28, 2012

### micromass

Well, of course the question is incorrect. The sets can not be equal. However, what the question asks is not whether the sets are equal, but whether they are isomorphic. You need to find an isomorphism between the sets.

5. Sep 28, 2012

### thoughtinknot

Okay the exponential map...

So consider ($\mathbb{R}$+, x) the group of positive real numbers, where x is normal multiplication. Then there exists a mapping, exp:$\mathbb{R}$$\rightarrow$$\mathbb{R}$+ such that exp(r)=er.

This can easily be shown to be an isomorphism, then I can use the cyclic group $\mathbb{I}$2 to extend this isomorphism to the negative reals aswell.

Last edited: Sep 28, 2012