Understanding the Unit Circle Group

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SUMMARY

The discussion focuses on the properties of the unit circle group, specifically the multiplication group \( U = \{ z \in \mathbb{C} : |z| = 1 \} \). It establishes that for any element \( z_0 \in U \), the set \( U z_0 \) forms a subgroup of \( U \). The second part of the problem addresses the isomorphism of the quotient group \( U / \langle -1 \rangle \), concluding that it is isomorphic to \( U \) itself, despite initial confusion regarding its relation to the half-circle or real numbers modulo 2.

PREREQUISITES
  • Understanding of group theory concepts, specifically subgroups and quotient groups.
  • Familiarity with complex numbers and their geometric interpretations on the unit circle.
  • Knowledge of isomorphisms and their implications in group theory.
  • Basic understanding of additive and multiplicative groups in relation to \( \mathbb{R} \) and \( \mathbb{C} \).
NEXT STEPS
  • Study the properties of subgroups in group theory, focusing on examples involving the unit circle.
  • Explore the concept of quotient groups and their applications in algebraic structures.
  • Investigate the relationship between multiplicative and additive groups, particularly in the context of \( \mathbb{R} \) and \( \mathbb{C} \).
  • Examine isomorphisms in detail, including practical examples and their significance in mathematical proofs.
USEFUL FOR

Mathematicians, students of abstract algebra, and anyone studying group theory, particularly those interested in the properties of the unit circle and its applications in complex analysis.

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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'.
 

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