Lagmental Vicfred

Kummer Extensions Come from Classes Modulo n-th Powers by Vicfred

When K contains the n-th roots of unity, adjoining an n-th root of a class can produce a cyclic extension. This is a compact note, but the quantifiers and hypotheses stay on the page.

The mathematical object

Kummer equations \(x^n=a\) describe cyclic extensions when roots of unity are available and \(\operatorname{char}K\nmid n\). In characteristic \(p\), Artin--Schreier equations \(x^p-x=a\) play the parallel role.

$$ \mu_n\subset K,\qquad L=K(\sqrt[n]{a}) $$

The first display fixes the mathematical data. I label it \(\mathsf{data}\) mentally, while the next is the \(\mathsf{claim}\); the bridge between them is the displayed \(\Longrightarrow\), not an automatic implication.

$$ [a]\in K^\times/(K^\times)^n $$

One explicit computation

Now evaluate one representative case. The result should agree with the structural law above, but it is obtained without assuming the conclusion.

$$ \sigma(\sqrt[n]{a})=\zeta_n\sqrt[n]{a},\qquad\operatorname{Gal}(L/K)\hookrightarrow\mu_n $$

Why the identity matters

The aligned summary deliberately puts the datum and conclusion on different rows. Mathematically, this is the distinction between specifying an object and proving a property of it.

$$ \begin{aligned} \mathsf{D}\;&:\quad \mu_n\subset K,\qquad L=K(\sqrt[n]{a}),\\[5pt] \mathsf{C}\;&:\quad [a]\in K^\times/(K^\times)^n. \end{aligned} $$

Where it can fail

Both theories have hypotheses that cannot be removed casually. Missing roots of unity or inseparability changes the Galois group and the classification.

$$ \boxed{\begin{gathered} \text{compact conclusion}\\[-2pt] [a]\in K^\times/(K^\times)^n \end{gathered}} $$

The final box is a summary, not a new assumption; the proof still lives in the definitions and the intervening calculation. The source keeps each scope delimiter visible for later inspection.

This article was posted on Sun 07 June 2015. Facts and circumstances may have changed since publication.
Please contact me before jumping to conclusions if something seems wrong or unclear.