# Finding the Norm of an element in a field extension

If I have a field extension of $\mathbb{Q}$ given by $\mathbb{Q}(\alpha)$ and the only thing I know about the primitive element $\alpha$ is it's minimal polynomial $p(x) = a_0 + a_1x + ... + x^n$ such that $p(\alpha) = 0$ how can I find the norm of an element $\beta \in \mathbb{Q}(\alpha)$?

I've got partial notes that seem to claim that I can define a linear map $T_\beta$ such that when I take the determinant of the associated matrix I'll end up with the norm of $\beta$ but I can't find the rest of my notes and I was hoping someone knows how to construct the matrix for $T_\beta$

EDIT: $\beta$ is given as $\beta = b_0 + b_1\alpha + ...$ with $\{\alpha^i\}$ forming a power basis for $\mathbb{Q}(\alpha)$

• Related (but slightly more specific): This question – ccorn May 5 '14 at 20:22
• @ccorn ; it's related but they've chosen a root of p($\alpha$) whereas I'm looking for a way to find the norm without making that choice. – AvatarOfChronos May 5 '14 at 20:27

Let $\beta = q(\alpha)$ where $q(X) = \sum_{i=0}^{n-1} b_i X^i$ and all $b_i\in\mathbb{Q}$. Represent $p(X) = \prod_{i=1}^n(X-\alpha_i)$ where the $\alpha_i$ are $\alpha$ and its conjugates. Then $\beta_i=q(\alpha_i)$ are $\beta$ and its conjugates. By definition of the norm, $$N(\beta) = \prod_{i=1}^n \beta_i = \prod_{i=1}^n q(\alpha_i) = \operatorname{Res}(p,q)$$ where $\operatorname{Res}$ is the polynomial resultant. It can be expressed solely in terms of the rational coefficients $a_0,\ldots,a_{n-1}$ of $p$ and $b_0,\ldots,b_{n-1}$ of $q$. It has indeed a representation as a determinant of a Sylvester matrix. It can be computed recursively in a way similar to the euclidean GCD algorithm. Writing out the resultant as a polynomial expression for general $q$ and $n\geq 3$ yields unwieldy expressions however.
• I'm trying to understand your answer so I've tried a concrete example. Let $\alpha$ solve $1 + x + x^2 + x^3 + x^4 == 0$ and then let $\beta \in \mathbb{Q}(\alpha)$ be given by $\beta = 1+23\alpha$. So from what I understand I have $q(X) = 1+23X$ and $p(X) = 1+X+X^2+X^3+X^4$ and I should look at Res($p,q$) which I get to be 1 (using mathematica to calculate the resultant of the polynomials defined above) whereas mathematica says "AlgebraicNumberNorm[1 + 23 $\alpha$]] = 268181". I think I've missed something in your answer. – AvatarOfChronos May 5 '14 at 20:24
• Pari/GP's polresultant(1+x+x^2+x^3+x^4,1+23*x) yields 268181. – ccorn May 5 '14 at 20:27
Think of the linear map from $\mathbb Q(\alpha)$ into itself given by $$T_\beta(\gamma) = \beta \gamma$$ the norm of $\beta$ is just the determinant of the matrix associated to this linear map, for instance if you write $$\beta \alpha^j = a_{0j} + a_{1j}\alpha + \dots + a_{(n-1)j} \alpha^{n-1}\quad \quad j = 0,1,\dots,n-1$$ then the matrix associated to $T_\beta$ relative to the base $\{1,\alpha,\dots,\alpha^{n-1}\}$, is $$M_\beta = \begin{pmatrix} a_{00} &\dots&a_{0(n-1)}\\\dots\\a_{(n-1)0} &\dots&a_{(n-1)(n-1)}\end{pmatrix}$$ you can see that the characteristic polynomial of this matrix is $$\det (M_\beta - XI_n) = (-1)^n\prod_{i=1}^n (X-\beta^{(i})$$ where $\beta^{(i}$ is the $i$-th conjugate of $\beta$. Putting $X=0$, we get that norm of $\beta$ in the extension $\mathbb Q(\alpha)/\mathbb Q$ is $$N_{\mathbb Q(\alpha)/\mathbb Q}\beta = \prod_{i=1}^n \beta^{(i} = \det M_\beta$$