Chapter 6: Problem 2
Show that it is possible to construct a regular 19 -gon with compass and marked ruler
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Chapter 6: Problem 2
Show that it is possible to construct a regular 19 -gon with compass and marked ruler
These are the key concepts you need to understand to accurately answer the question.
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If \(f(x)\) is an irreducible cubic equation over the field \(F \subseteq \mathbb{R}\) with one real and two complex roots, then its Galois group is \(S_{3}\)
The discriminant. Let \(f(x)\) be an irreducible cubic polynomial with coefficients in the field \(F \subseteq \mathbb{R},\) and let its roots be \(\alpha_{1}, \alpha_{2}, \alpha_{3}\) in its splitting field. We define the \(d i s-\) criminant of \(f(x)\) to be $$\Delta=\left(\alpha_{1}-\alpha_{2}\right)^{2}\left(\alpha_{1} \alpha_{3}\right)^{2}\left(\alpha_{2}-\alpha_{3}\right)^{2}$$ (a) Show that \(\Delta \in F\) (b) Show that \(\Delta>0\) if and only if \(f(x)\) has 3 real roots, while \(\Delta<0\) if and only if \(f(x)\) has one real and two complex roots. (c) Show that the Galois group of \(f(x)\) is \(\mathbb{Z}_{3}\) if and only if \(\sqrt{\Delta} \in F\); otherwise, it is \(S_{3}\)
Prove the following theorem of Hölder: Let \(f(x)\) be irreducible of degree \(n\) over \(\mathbb{Q}\), having all real roots. If at least one of these roots can be expressed by real radicals (of various degrees), then \(n=2^{k},\) and all the roots can be expressed by real square roots.
We have seen (Theorem 28.4 ) that \(\alpha=2 \cos 20^{\circ}\) is a root of the equation \(x^{3}-3 x-1=0 .\) Show that the other two roots of this equation are contained in the field \(\mathbb{Q}(\alpha),\) and so its Galois group is \(\mathbb{Z}_{3}\)
Suppose, in addition to the compass and marked ruler, we were given tools to extract 5 th roots and quintisect angles. Show that even with these new tools, it is still not possible to solve a general fifth-degree equation. Hint: Show that the Galois group of any extension obtained with these new tools is still solvable, and then take a quintic equation with Galois group \(S_{5},\) such as Example 32.4 .4
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