Let p be a prime, $\xi_p \in \mathbb{C}$ a primitive p-th unit root and $K = \mathbb{Q}(\xi_p)$.
Give a normal basis for $K/\mathbb{Q}$.
I know, that a basis of $L/K$ (finite and galois) is called a normal basis if it has the form $\{\sigma(a)\}_{\sigma \in \text{Gal}(L/K)}$ for an $a \in L$. A procedure to determine such an element a if $|K| = \infty$ is the following:
- Determine the minimal polynomial $m_{\beta}(X)$ for a primitive Element $\beta$ ($L = K(\beta)$)
- Set $g_{\sigma}(X) := \prod\limits_{\substack{\tau \in G\\\tau \neq \sigma}} (X - \tau(\beta))$ and $A(X) := (g_{\tau^{-1}\sigma})_{\tau,\sigma \in G}$ with $G := \text{Gal}(L/K)$
- Search a $\gamma \in K$ with $det(A(\gamma)) \neq 0$
- Set $a := \frac{m_{\beta}(\gamma)}{\gamma - \beta}$
So far so good. I have only been able to achieve that $\Phi_p(X) = \frac{X^p - 1}{X-1} = X^{p-1} + . . . + X + 1$ is the minimal polynomial of $\xi_p$ over $\mathbb{Q}$ since p is prime. The elements of the Galois group $G:=\text{Gal}(K/\mathbb{Q})$ should affect the primitive root of unit $\beta$ like potencies. That means it exists an isomorphism \begin{align} k : G \longrightarrow (\mathbb{Z}/n\mathbb{Z})^x\\ \sigma \mapsto \kappa(\sigma) + n\mathbb{Z} \end{align} so that $\sigma(w) = w^{\kappa(\sigma)}$. [$(K)^x$ means the group of invertible units in K]
Now I'm unsure how to proceed. Thanks for help.