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Silversonic
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Homework Statement
Let R be a subring of ℂ such that the group of invertible elements U(R) is finite, show that this group is a cyclic group. (Group operation being multiply).
Homework Equations
The Attempt at a Solution
I have the answer, and I got very close to getting there myself before I reached a roadblock and couldn't move on. Even though I've looked at the answer, I still cannot understand the part that was blocking me. I've genuinely spent a long time trying to understand this last part and it's probably something really simple, which has annoyed me a great deal.
The deal is this;
The group is finite, and therefore any element must have a modulus of one - because if an elements didn't have modulus one any power of said element would be a unique, and thus the group would not be finite. Hence every element of the group of invertile elements has the form
[itex] e^{2\pi i \frac {p}{q}} [/itex] where p and q are natural numbers, i.e. their arguments are rational multiples of two pi.
The step the answer takes is this. Assume that q (above) is the lowest common denominator of all the fractions [itex] \frac {p}{q} [/itex] that occur. Then there will be an element [itex] x [/itex] within the group such that
[itex] x = e^{2\pi i \frac {p_0}{q}} [/itex], where [itex] \frac {p_0}{q} [/itex] is in lowest terms, i.e. they are coprime. The proof carries on and I am able to understand the rest.
But the bolded bit, I've embarassingly spent ages on this and don't know how this can be true. Every rational number in the exponent was re-written in the form where the denominator was the LCD of all the exponents of the elements in the group, I understand that. But I don't see how it goes to show that one of the elements is coprime to this said LCD (and as such, was already in the form [itex] \frac {p}{q} [/itex] where q is the LCD, to begin with).
Would it involve writing [itex] p = kq +r [/itex] (r<q) so that every element becomes of the form;
[itex] e^{2\pi i \frac {r}{q}} [/itex] and showing that a combination of elements such as this can produce a [itex] p_0 [/itex] which is coprime to [itex] q [/itex]?
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