Solving Galois Group Problem Q7: Step-by-Step Guide

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In summary, the Leibniz formula for the determinant of a matrix states that the sum of all signed terms equals the product of all the corresponding Sigma values. This is due to the fact that the Galois group of the matrix is the group of automorphisms of the matrix that fix the vector in its domain. The group of automorphisms of M/Q is the group of automorphisms of M that fix Q, and the group of automorphisms of K/Q is the group of automorphisms of K that fix M. The permutations of the Sigma values affect only the signs of the terms, and the terms are invariant under Galois group of M/Q.
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Hello Kitty
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I am trying to solve question 7 on this problem sheet. This is my progress so far:

Using the Leibniz formula for the determinant of a matrix I deduce that

[tex] \det(\Omega) = \sum_{\tau \in S_n} sign(\tau) \prod_{i = 1}^n \sigma_i (w_{\tau(i)}) [/tex]Hence [tex] P + N = \sum_{\tau \in S_n}\prod_{i = 1}^n \sigma_i (w_{\tau(i)}) [/tex]

and [tex] PN = (\sum_{\tau \in A_n}\prod_{i = 1}^n \sigma_i (w_{\tau(i)}))\cdot (\sum_{\tau \in S_n \setminus A_n}\prod_{i = 1}^n \sigma_i (w_{\tau(i)})) [/tex]

[tex] = \sum_{\tau \in S_n; \pi \in S_n \setminus A_n}\prod_{i = 1}^n \sigma_i (w_{\tau(i)}w_{\pi(i)}) [/tex]

Having not done a formal course in Galois Theory, I'm a bit unsure about the significance of M here. I realize that Aut(M/Q) can only be called Gal(M/Q) if M:Q is normal (implied by it's being a splitting field) and separable (automatic in C).

So Gal(M/Q) is a the group of automorphisms of M that fix Q and thus due to the Galois Correspondence, the subgroup of Gal(M/Q) of elements that fix K corresponds uniquely to K and vice versa. I don't quite see how this helps here though.

Since

[tex] Aut(K/Q) = \{\sigma_1 , \ldots , \sigma_n \} [/tex]

I see that applying any of these to the above expressions would leave them invariant, but what I don't see is why they are invariant under Gal(M/Q). After all M seems to be arbitrary.
 

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  • #2
Hello Kitty said:
[tex] Aut(K/Q) = \{\sigma_1 , \ldots , \sigma_n \} [/tex]

No, I don't think that's right. I think that [itex]\sigma_i[/itex] are the embeddings of K into C. Note that, as M is normal and contains K, they are also embeddings into M and are permuted by applying the elements of the Galois group of M/Q. The question can then be approached by considering what happens to P and N under even and odd permutations.
 
  • #3
OK thanks. Yeah the \sigma 's are the embeddings of K. The only thing about what you say that doesn't make sense to me is the bit about them being permuted by the elements of the Galois group of M/Q. The elements of this group permute the Q-automorphisms of M by definition, but I don't see how this implies that they permute the embeddings of K.
 
  • #4
each embedding sigma is a homorphism K->C, whose image will in fact be in M, so can be regarded as a homomorphism K->M. An element g of the Galois group of M/Q is a homomorphism M->M. So, by composition, [itex]g\sigma:K->M[/itex] which is just another embedding of K into M (and therefore, into C).
 
  • #5
Thanks - excellently explained!
 

FAQ: Solving Galois Group Problem Q7: Step-by-Step Guide

What is the Galois group problem?

The Galois group problem is a mathematical problem that involves determining the symmetry group of a polynomial equation. This group, also known as the Galois group, represents the set of all possible permutations of the roots of the polynomial equation that preserve its algebraic structure.

What is the significance of solving the Galois group problem?

Solving the Galois group problem has important implications in the fields of algebra, number theory, and cryptography. It allows us to understand the structure of a polynomial equation and its solutions, and can be used to determine whether a polynomial equation is solvable by radicals or not.

How do you approach solving the Galois group problem?

The first step in solving the Galois group problem is to identify the field of coefficients and the field of roots of the polynomial equation. Then, you can use a variety of techniques such as finding the discriminant, using Galois theory, or constructing a Galois extension to determine the Galois group. Finally, you can use group theory to analyze the structure of the Galois group and find its subgroups.

What is a step-by-step guide for solving the Galois group problem?

A step-by-step guide for solving the Galois group problem would include the following steps: 1. Identify the polynomial equation and its field of coefficients. 2. Determine the field of roots of the polynomial equation. 3. Find the discriminant of the polynomial equation. 4. Use Galois theory to determine the Galois group. 5. Analyze the structure of the Galois group using group theory. 6. Find the subgroups of the Galois group. 7. Use the subgroups to determine the solutions of the polynomial equation.

Are there any resources available for solving the Galois group problem?

Yes, there are many resources available for solving the Galois group problem. These include textbooks on Galois theory, online tutorials and videos, and specialized software programs. It is also helpful to consult with experts in the field or to join online communities where you can discuss and learn about different approaches to solving the problem.

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