Galois Extensions: Homework Analysis

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Homework Help Overview

The discussion revolves around determining whether certain field extensions involving the fourth root of 5 and imaginary units are Galois over the rational numbers. The original poster presents specific extensions: \(\mathbb{Q}(\sqrt{-5})\), \(\mathbb{Q}(\beta + i\beta)\), and their relationships to each other and to \(\mathbb{Q}\).

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the conditions for a field to be Galois, discussing the concept of splitting fields and distinct roots. There are attempts to apply these concepts to the given extensions, with some questioning the nature of \(\mathbb{Q}(\beta + i\beta)\) and its relation to other fields.

Discussion Status

Some participants have provided insights regarding the Galois nature of \(\mathbb{Q}(\sqrt{-5})\) and its polynomial representation. However, there remains uncertainty about the other extensions, with participants expressing difficulty in identifying explicit separable polynomials and questioning their Galois status.

Contextual Notes

Participants note challenges in identifying the relationships between the fields and the polynomials that would demonstrate their Galois properties. There is an acknowledgment of the complexity involved in proving or disproving the Galois nature of the extensions under discussion.

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Homework Statement



Let \beta be a real, positive fourth root of 5. Is \mathbb{Q}(\sqrt{-5}) Galois over \mathbb{Q}? How about \mathbb{Q}(\beta + i\beta)/\mathbb{Q} or \mathbb{Q}(\beta + i\beta)/\mathbb{Q}(\sqrt{-5})?


Homework Equations



An extension is Galois when it is normal and separable.

The Attempt at a Solution



Any push whatsoever in the right direction would be much appreciated.
 
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Can you tell me anything at all about those extensions?
 
Do you know a condition equivalent to being Galois that involves splitting?
 
well, a field A is galois over B if it is the splitting field of some polynomial in B[x] which has distinct roots in A.

so then, \mathbb{Q}(\sqrt{-5}) is the splitting field of the polynomial x^2 + 5, which has coefficients in \mathbb{Q}. this is because x^2 + 5 = (x + \sqrt{-5})(x - \sqrt{-5}), and hence the roots are also distinct, and so in the first case, we do have a galois extension.

but i can't apply this to the second two ideas, because i can't tell anything about the field \mathbb{Q}(\beta + i\beta). it doesn't seem to be the same as \mathbb{Q}(i, \beta) or anything of the sort. am i missing something obvious?

thanks so much.
 
also, thinking about the polynomial (x + \beta + i\beta)(x + \beta - i\beta)(x - \beta + i\beta)(x - \beta - i\beta) doesn't help, because it gives me the information that \mathbb{Q}(\beta + i\beta) is galois over \mathbb{Q}(\sqrt{5}), but nothing else, and i don't see how this helps in the problem.
 
Last edited:
shoplifter said:
also, thinking about the polynomial (x + \beta + i\beta)(x + \beta - i\beta)(x - \beta + i\beta)(x - \beta - i\beta) doesn't help, because it gives me the information that \mathbb{Q}(\beta + i\beta) is galois over \mathbb{Q}(\sqrt{5}), but nothing else, and i don't see how this helps in the problem.

Is b-ib really in Q(b+ib)?
 
no, it isn't, but it doesn't follow that it's not galois, right? I've worked for a while, and can't find explicit separable polynomials whose splitting fields are precisely those, but then what does that prove?

i really don't see a way to disprove they're galois extensions. even if they are, i can't find the explicit polynomials, but my intuition points to a disproof. =( but how do i do that?
 
oh i got the fact that it's galois over the third one -- from the polynomial x^2 - 2i*b^2.

what about the last one?
 

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