# Homework Help: Isomorphism from group to a product group

1. Nov 13, 2011

### EV33

1. The problem statement, all variables and given/known data

Determine whether or not G is isomorphic to the product group HXK.

G=ℂx
H={unit circle}
K={Positive real numbers}

2. Relevant equations

Let H and K be subroups of G, and let fXK→G be the multiplication map, defined by f(h,k)=hk. Its image is the set HK={hk: h in H, k in K}.

f is an isomorphism from the product group HXK to G iff H intersect K is the identity,HK=G, and also H and K are normal subgroups of G.

3. The attempt at a solution

My actual question is how am I supposed to look at the unit circle?
I know if I view it as all the points (a,b) on the unit circle then its intersection with K would be the empty set and thus G wouldn't be isomorphic to HXK.

but I have a feeling that is too simple. Am I potentially supposed to view the unit circle as cos(θ)+i*sin(θ) where θ is in ℝ?

Last edited: Nov 13, 2011
2. Nov 13, 2011

### I like Serena

Any complex number in ℂx can be represented as re and the reverse holds as well.
So z → (r,θ) is a bijection.
If you define the operations in HxK to be regular multiplication, respectively regular addition, you have preservation of the operation, since it literally is the same.