Understanding the Unit Circle Group

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[SOLVED] unit circle

Homework Statement


My book contains the following problem:

Let U be the multiplication group \{z \in C : |z| = 1\}

1) Let z_0 be in U. Show that U z_0 = \{ z z_0 : z \in U \} is a subgroup of U, and compute U mod U z_0.
2) To what group is U/<-1> isomorphic to?

Homework Equations


The Attempt at a Solution


I think 1) is so insanely trivial it is not worth asking. The answer is clearly the trivial group, right?

My book says that the answer to 2) is U, but it seems it should be the half-circle or the reals mod 2 or something. Why is it U?\in\in
 
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U can also be represented as an additive group. It's R/(Z*2pi). U/<-1> is R/(Z*pi). Is there any real difference? The 'half circle' is isomorphic to the 'full circle'.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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