TransWikia.com

Galois group Abstract algebra

Mathematics Asked by user462999 on January 18, 2021

$omega$ is the primitive $4^{th}$ root of unity, then find $Galleft(mathbb{Qleft(sqrt{2},omegaright)/mathbb{Q}}right).$

First i calculate order of $Galleft(mathbb{Qleft(sqrt{2},omegaright)/mathbb{Q}}right)$ which is $4$ by use of minimal polynomial.

$left[mathbb{Q}left(sqrt{2},omegaright):mathbb{Q}right]=left[mathbb{Q}left(sqrt{2},omegaright):mathbb{Q}left(omegaright)right]left[mathbb{Q}left(omegaright):mathbb{Q}right]$

$left[mathbb{Q}left(sqrt{2},omegaright):mathbb{Q}left(omegaright)right]=2$

$left[mathbb{Q}left(omegaright):mathbb{Q}right]=2$ because $x^2+1$ is minimal polynomial.

Therefore $left[mathbb{Q}left(sqrt{2},omegaright):mathbb{Q}right]=2times 2=4$.

(I am not sure that i calculate right)

I don’t know how elements of $Galleft(mathbb{Q}left(sqrt{2},omegaright)/mathbb{Q}right)$ look like.

I think it is of the type

$varepsilon=omega rightarrow omega,~sqrt{2}rightarrow sqrt{2}$

$alpha=omegarightarrow omega^2,~sqrt{2}rightarrow sqrt{2}$

$beta=omegarightarrow omega^3,~sqrt{2}rightarrow sqrt{2}$

$gamma=omega rightarrow omega,~sqrt{2}rightarrow omegasqrt{2}$.

Four elements complete here and i think i did wrong.

Any hint how to find element.

Thank you.

One Answer

$mathbb Q(sqrt 2, omega)$ is the compositum of two Galois fields extensions, $mathbb Q(sqrt 2)$ and $Q(omega)$ and hence also Galois over $mathbb Q$.

Note that $mathbb Q(sqrt 2) cap mathbb Q(omega) =mathbb Q$.

This induces a group homomorphism.

$$operatorname{Gal}(mathbb Q(sqrt 2, omega)/ mathbb Q)rightarrow operatorname{Gal}(mathbb Q(sqrt 2)/mathbb Q) times operatorname{Gal}(mathbb Q(omega )/mathbb Q)cong mathbb Z_2 times mathbb Z_2 $$ $$sigmamapsto (sigma|_{mathbb Q(sqrt 2)}, sigma|_{mathbb Q(omega)} ) $$

This is injective and hence by cardinality arguments, an isomorphism. So we get $operatorname{Gal}(mathbb Q(sqrt 2, omega)/ mathbb Q) cong mathbb Z_2 times mathbb Z_2 $

Answered by Baidehi on January 18, 2021

Add your own answers!

Ask a Question

Get help from others!

© 2024 TransWikia.com. All rights reserved. Sites we Love: PCI Database, UKBizDB, Menu Kuliner, Sharing RPP