# Thread: Cyclotomic Fields & Bases....

1. ## Cyclotomic Fields & Bases....

Hi everyone! :-)

Suppose Zeta is any primitive 5th root of unity.

Then we know that Q(Zeta) has a basis {1,Zeta,Zeta^2,Zeta^3} because Zeta is algebraic over Q.

But how can I show that {Zeta,Zeta^-1,Zeta^2,Zeta^-2} is a basis? :-s

Also, how does the Galois Group of Q(Zeta) act on this new basis?

Furthermore, how can you prove the existence of a unique subfield E such that [Q(Zeta):E]=2, & find all a,b,c,d such that aZeta+bZeta^-1+cZeta^2+dZeta^-2 is in E but not Q? (apparently you can then show that E = Q(Sqrt(5)) !! :-o )

I have absolutely no idea where to begin! :-s

Any help on ANY of these questions would be amazing! :-)

Thank you! x

2. Originally Posted by AAM
Hi everyone! :-)
You like to smile a lot, .

Suppose Zeta is any primitive 5th root of unity.

Then we know that Q(Zeta) has a basis {1,Zeta,Zeta^2,Zeta^3} because Zeta is algebraic over Q.

But how can I show that {Zeta,Zeta^-1,Zeta^2,Zeta^-2} is a basis? :-s
Remember that $\zeta^{-1} = \zeta^4$ and $\zeta^{-2} = \zeta^3$.

Also, how does the Galois Group of Q(Zeta) act on this new basis?
The Galois group, $G=\text{Gal}(\mathbb{Q}(\zeta)/\mathbb{Q})$ is isomorphic to the cyclic group $\mathbb{Z}_4$. Let $\sigma : \mathbb{Q}(\zeta) \to \mathbb{Q}(\zeta)$ be the automorphism defined by $\sigma (\zeta) = \zeta^2$. Then $G = \left< \sigma \right>$. Since you know $\sigma (\zeta) = \zeta^2$ you can determine what $\sigma (\zeta^k)$ is for $k=2,3,4$.

Furthermore, how can you prove the existence of a unique subfield E such that [Q(Zeta):E]=2, & find all a,b,c,d such that aZeta+bZeta^-1+cZeta^2+dZeta^-2 is in E but not Q? (apparently you can then show that E = Q(Sqrt(5)) !! :-o )
$\zeta - \zeta^2 - \zeta^3 + \zeta^4= \sqrt{5}$
Therefore, $E = \mathbb{Q}(\sqrt{5})$ is a subfield of $\mathbb{Q}(\zeta)$.