Techniques of Solving Equations à la Galois


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Chapter3

    Exhausting! Packed with Cyclic Extensions!

\(\qquad \qquad \qquad f(x)=x^4+4x+2 \qquad Galois \ Group:S_4\)

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[3-5] Verifying that the maps \(\rho_i\) carry a group structure

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Since we have established that \(F_0(v)\) is a Galois extension of \(F_0\), the following theorem of Galois theory applies.

“There exist as many field automorphisms of \(F_0(v)\) as there are conjugate roots of \(g_0(x)\), each sending the primitive element \(v\) to a conjugate root; moreover, these automorphisms form a group.”

In this section we verify that this theorem holds here.
To that end, we take \(\{\rho_i\}\ [i=1,2,\ldots,24]\) defined in (5.1) as candidates for such automorphisms. Equation (5.1) regards the polynomial expressions of \(v_i\) from (4.10–13) as the images of \(v\) under \(\rho_i\). We then check that the \(\rho_i\) form a group isomorphic to the symmetric group \(S_4\), and that each \(\rho_i\) is an automorphism mapping conjugate roots to conjugate roots.

\begin{align} \rho_{1}(v) &\equiv v_1=v \notag \\ \notag \\ \rho_{2}(v) &\equiv v_2=\frac{801167701943012874015343807}{2531636596706154203109159208286706204672}v^{23}-\frac{51207699710669004924125}{99737485589022345786910893443907584}v^{22} \notag \\ &\quad .......... \notag \\ &+\frac{4663968749846627680850109200993107}{2971516062464968406218495540075104}v+\frac{2718803338720300088760700554765}{1521873254227025539961408896544} \\ \notag \\ &\qquad \qquad .........\notag \\ \notag \\ \rho_{23}(v) &\equiv v_{23}=-\frac{801167701943012874015343807}{2531636596706154203109159208286706204672}v^{23}-\frac{51207699710669004924125}{99737485589022345786910893443907584}v^{22} \notag \\ &\quad .......... \notag \\ &-\frac{4663968749846627680850109200993107}{2971516062464968406218495540075104}v+\frac{2718803338720300088760700554765}{1521873254227025539961408896544} \notag \\ \notag \\ \rho_{24}(v) &\equiv v_{24}=-v \notag \\ \end{align}



We must confirm that \(\rho_i(v)\), \(i=1,2,\ldots,24\), form a group under composition. For this, we compile the multiplication table \(\rho_i \circ \rho_j = \rho_k\) and check whether the compositions yield a group. The verification procedure is exactly the same as in Chapter 2, Section [2-6]. In Chapter 3 the formulas for \(v_i\) are too long to present the intermediate computations, so we simply display the resulting table below .

From this table we find that the maps \(\rho_i\) form a group under composition that is equivalent to the symmetric group \(S_4\).

[Table 3-1] Multiplication table for \(S_4\): \(\rho_i \circ \rho_j =\rho_k\) (entries shown as numbers only)
\( i \backslash j \)\(1 \)\(2 \)\(3 \)\(4 \)\(5 \)\(6 \)\(7 \)\(8 \)\(9 \)\({10} \)\({11} \)\({12} \)\({13} \)\({14} \)\({15} \)\({16} \)\({17} \)\({18} \)\({19} \)\({20} \)\({21} \)\({22} \)\({23} \)\({24}\)
\(1\)\(1 \)\(2 \)\(3 \)\(4 \)\(5 \)\(6 \)\(7 \)\(8 \)\(9 \)\({10} \)\({11} \)\({12} \)\({13} \)\({14} \)\({15} \)\({16} \)\({17} \)\({18} \)\({19} \)\({20} \)\({21} \)\({22} \)\({23} \)\({24}\)
\(2\)\(2 \)\(1 \)\(5 \)\(6 \)\(3 \)\(4 \)\(8 \)\(7 \)\({11} \)\({12} \)\(9 \)\({10} \)\({19} \)\({20} \)\({21} \)\({22} \)\({23} \)\({24}\)\({13} \)\({14} \)\({15} \)\({16} \)\({17} \)\({18}\)
\(3 \)\(3 \)\(4 \)\(1 \)\(2 \)\(6 \)\(5 \)\({13} \)\({14} \)\({15} \)\({16} \)\({17} \)\({18} \)\(7 \)\(8 \)\(9 \)\({10} \)\({11} \)\({12} \)\({20} \)\({19} \)\({23} \)\({24}\)\({21} \)\({22} \)
\(4\)\(4 \)\(3 \)\(6 \)\(5 \)\(1 \)\(2 \)\({14} \)\({13} \)\({17} \)\({18} \)\({15} \)\({16} \)\({20} \)\({19} \)\({23} \)\({24}\)\({21} \)\({22} \)\(7 \)\(8 \)\(9 \)\({10} \)\({11} \)\({12} \)
\(5 \)\(5 \)\(6 \)\(2 \)\(1 \)\(4 \)\(3 \)\({19} \)\({20} \)\({21} \)\({22} \)\({23} \)\({24}\)\(8 \)\(7 \)\({11} \)\({12} \)\(9 \)\({10} \)\({14} \)\({13} \)\({17} \)\({18} \)\({15} \)\({16} \)
\(6 \)\(6 \)\(5 \)\(4 \)\(3 \)\(2 \)\(1 \)\({20} \)\({19} \)\({23} \)\({24}\)\({21} \)\({22} \)\({14} \)\({13} \)\({17} \)\({18} \)\({15} \)\({16} \)\(8 \)\(7 \)\({11} \)\({12} \)\(9 \)\({10} \)
\(7 \)\(7 \)\(8 \)\(9 \)\({10} \)\({11} \)\({12} \)\(1 \)\(2 \)\(3 \)\(4 \)\(5 \)\(6 \)\({15} \)\({16} \)\({13} \)\({14} \)\({18} \)\({17} \)\({21} \)\({22} \)\({19} \)\({20} \)\({24}\)\({23} \)
\(8 \)\(8 \)\(7 \)\({11} \)\({12} \)\(9 \)\({10} \)\(2 \)\(1 \)\(5 \)\(6 \)\(3 \)\(4 \)\({21} \)\({22} \)\({19} \)\({20} \)\({24}\)\({23} \)\({15} \)\({16} \)\({13} \)\({14} \)\({18} \)\({17} \)
\(9 \)\(9 \)\({10} \)\(7 \)\(8 \)\({12} \)\({11} \)\({15} \)\({16} \)\({13} \)\({14} \)\({18} \)\({17} \)\(1 \)\(2 \)\(3 \)\(4 \)\(5 \)\(6 \)\({22} \)\({21} \)\({24}\)\({23} \)\({19} \)\({20} \)
\({10} \)\({10} \)\(9 \)\({12} \)\({11} \)\(7 \)\(8 \)\({16} \)\({15} \)\({18} \)\({17} \)\({13} \)\({14} \)\({22} \)\({21} \)\({24}\)\({23} \)\({19} \)\({20} \)\(1 \)\(2 \)\(3 \)\(4 \)\(5 \)\(6 \)
\({11} \)\({11} \)\({12} \)\(8 \)\(7 \)\({10} \)\(9 \)\({21} \)\({22} \)\({19} \)\({20} \)\({24}\)\({23} \)\(2 \)\(1 \)\(5 \)\(6 \)\(3 \)\(4 \)\({16} \)\({15} \)\({18} \)\({17} \)\({13} \)\({14} \)
\({12} \)\({12} \)\({11} \)\({10} \)\(9 \)\(8 \)\(7 \)\({22} \)\({21} \)\({24}\)\({23} \)\({19} \)\({20} \)\({16} \)\({15} \)\({18} \)\({17} \)\({13} \)\({14} \)\(2 \)\(1 \)\(5 \)\(6 \)\(3 \)\(4 \)
\({13} \)\({13} \)\({14} \)\({15} \)\({16} \)\({17} \)\({18} \)\(3 \)\(4 \)\(1 \)\(2 \)\(6 \)\(5 \)\(9 \)\({10} \)\(7 \)\(8 \)\({12} \)\({11} \)\({23} \)\({24}\)\({20} \)\({19} \)\({22} \)\({21} \)
\({14} \)\({14} \)\({13} \)\({17} \)\({18} \)\({15} \)\({16} \)\(4 \)\(3 \)\(6 \)\(5 \)\(1 \)\(2 \)\({23} \)\({24}\)\({20} \)\({19} \)\({22} \)\({21} \)\(9 \)\({10} \)\(7 \)\(8 \)\({12} \)\({11} \)
\({15} \)\({15} \)\({16} \)\({13} \)\({14} \)\({18} \)\({17} \)\(9 \)\({10} \)\(7 \)\(8 \)\({12} \)\({11} \)\(3 \)\(4 \)\(1 \)\(2 \)\(6 \)\(5 \)\({24}\)\({23} \)\({22} \)\({21} \)\({20} \)\({19} \)
\({16} \)\({16} \)\({15} \)\({18} \)\({17} \)\({13} \)\({14} \)\({10} \)\(9 \)\({12} \)\({11} \)\(7 \)\(8 \)\({24}\)\({23} \)\({22} \)\({21} \)\({20} \)\({19} \)\(3 \)\(4 \)\(1 \)\(2 \)\(6 \)\(5 \)
\({17} \)\({17} \)\({18} \)\({14} \)\({13} \)\({16} \)\({15} \)\({23} \)\({24}\)\({20} \)\({19} \)\({22} \)\({21} \)\(4 \)\(3 \)\(6 \)\(5 \)\(1 \)\(2 \)\({10} \)\(9 \)\({12} \)\({11} \)\(7 \)\(8 \)
\({18} \)\({18} \)\({17} \)\({16} \)\({15} \)\({14} \)\({13} \)\({24}\)\({23} \)\({22} \)\({21} \)\({20} \)\({19} \)\({10} \)\(9 \)\({12} \)\({11} \)\(7 \)\(8 \)\(4 \)\(3 \)\(6 \)\(5 \)\(1 \)\(2 \)
\({19} \)\({19} \)\({20} \)\({21} \)\({22} \)\({23} \)\({24}\)\(5 \)\(6 \)\(2 \)\(1 \)\(4 \)\(3 \)\({11} \)\({12} \)\(8 \)\(7 \)\({10} \)\(9 \)\({17} \)\({18} \)\({14} \)\({13} \)\({16} \)\({15} \)
\({20} \)\({20} \)\({19} \)\({23} \)\({24}\)\({21} \)\({22} \)\(6 \)\(5 \)\(4 \)\(3 \)\(2 \)\(1 \)\({17} \)\({18} \)\({14} \)\({13} \)\({16} \)\({15} \)\({11} \)\({12} \)\(8 \)\(7 \)\({10} \)\(9 \)
\({21} \)\({21} \)\({22} \)\({19} \)\({20} \)\({24}\)\({23} \)\({11} \)\({12} \)\(8 \)\(7 \)\({10} \)\(9 \)\(5 \)\(6 \)\(2 \)\(1 \)\(4 \)\(3 \)\({18} \)\({17} \)\({16} \)\({15} \)\({14} \)\({13} \)
\({22} \)\({22} \)\({21} \)\({24}\)\({23} \)\({19} \)\({20} \)\({12} \)\({11} \)\({10} \)\(9 \)\(8 \)\(7 \)\({18} \)\({17} \)\({16} \)\({15} \)\({14} \)\({13} \)\(5 \)\(6 \)\(2 \)\(1 \)\(4 \)\(3 \)
\({23} \)\({23} \)\({24}\)\({20} \)\({19} \)\({22} \)\({21} \)\({17} \)\({18} \)\({14} \)\({13} \)\({16} \)\({15} \)\(6 \)\(5 \)\(4 \)\(3 \)\(2 \)\(1 \)\({12} \)\({11} \)\({10} \)\(9 \)\(8 \)\(7 \)
\({24}\)\({24}\)\({23} \)\({22} \)\({21} \)\({20} \)\({19} \)\({18} \)\({17} \)\({16} \)\({15} \)\({14} \)\({13} \)\({12} \)\({11} \)\({10} \)\(9 \)\(8 \)\(7 \)\(6 \)\(5 \)\(4 \)\(3 \)\(2 \)\(1 \)


In Chapter 2 we also listed a table for \(\rho_i(v_j)\). The content is identical here, and reading it with the indices relabeled by the numbers in the table above is sufficient. In conclusion, [Table 3-1] shows that the maps \(\rho_i\) form a group equivalent to the symmetric group \(S_4\). Moreover, for every \(v_j\) we have a mapping relation of the form \(\rho_i(v_j)=v_k\).
Hence the candidates \(\{\rho_i\}\) are precisely the elements of the Galois group \(Gal(F_0(v)/F_0)\).

[3-6] The action of \(\rho_i\) on \(\{\alpha,\beta,\gamma,\delta\}\)

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We now compute how \(\{ \rho_1,\rho_2,\ldots,\rho_{24}\}\) act on the four roots. Again, the expressions become unwieldy, so we omit the intermediate steps; the method is exactly the same as in Chapter 2.
[Table 3-2] records how each \(\rho_i\) acts on \([\alpha,\beta,\gamma,\delta]\).
We find that the action of \(\rho_i\) coincides exactly with the permutation action of the elements \(\sigma_i\) of \(S_4\) in (2.2) on \([\alpha,\beta,\gamma,\delta]\).

[Table 3-2] Transformation table for \(\rho_i\) (1)
\(\rho_i(\alpha)\)\(\rho_i(\beta)\)\(\rho_i(\gamma)\)\(\rho_i(\delta)\)
\(\rho_{1}\)\(\alpha\)\(\beta\)\(\gamma\)\(\delta\)
\(\rho_{2}\)\(\alpha\)\(\beta\)\(\delta\)\(\gamma\)
\(\rho_{3}\)\(\alpha\)\(\gamma\)\(\beta\)\(\delta\)
\(\rho_{4}\)\(\alpha\)\(\gamma\)\(\delta\)\(\beta\)
\(\rho_{5}\)\(\alpha\)\(\delta\)\(\beta\)\(\gamma\)
\(\rho_{6}\)\(\alpha\)\(\delta\)\(\gamma\)\(\beta\)
\(\rho_{7}\)\(\beta\)\(\alpha\)\(\gamma\)\(\delta\)
\(\rho_{8}\)\(\beta\)\(\alpha\)\(\delta\)\(\gamma\)

[Table 3-2] Transformation table for \(\rho_i\) (2)
\(\rho_i(\alpha)\)\(\rho_i(\beta)\)\(\rho_i(\gamma)\)\(\rho_i(\delta)\)
\(\rho_{9}\)\(\beta\)\(\gamma\)\(\alpha\)\(\delta\)
\(\rho_{10}\)\(\beta\)\(\gamma\)\(\delta\)\(\alpha\)
\(\rho_{11}\)\(\beta\)\(\delta\)\(\alpha\)\(\gamma\)
\(\rho_{12}\)\(\beta\)\(\delta\)\(\gamma\)\(\alpha\)
\(\rho_{13}\)\(\gamma\)\(\alpha\)\(\beta\)\(\delta\)
\(\rho_{14}\)\(\gamma\)\(\alpha\)\(\delta\)\(\beta\)
\(\rho_{15}\)\(\gamma\)\(\beta\)\(\alpha\)\(\delta\)
\(\rho_{16}\)\(\gamma\)\(\beta\)\(\delta\)\(\alpha\)

[Table 3-2] Transformation table for \(\rho_i\) (3)
\(\rho_i(\alpha)\)\(\rho_i(\beta)\)\(\rho_i(\gamma)\)\(\rho_i(\delta)\)
\(\rho_{17}\)\(\gamma\)\(\delta\)\(\alpha\)\(\beta\)
\(\rho_{18}\)\(\gamma\)\(\delta\)\(\beta\)\(\alpha\)
\(\rho_{19}\)\(\delta\)\(\alpha\)\(\beta\)\(\gamma\)
\(\rho_{20}\)\(\delta\)\(\alpha\)\(\gamma\)\(\beta\)
\(\rho_{21}\)\(\delta\)\(\beta\)\(\alpha\)\(\gamma\)
\(\rho_{22}\)\(\delta\)\(\beta\)\(\gamma\)\(\alpha\)
\(\rho_{23}\)\(\delta\)\(\gamma\)\(\alpha\)\(\beta\)
\(\rho_{24}\)\(\delta\)\(\gamma\)\(\beta\)\(\alpha\)


Strictly speaking, each \(\rho_i\) was defined to send \(v\) to a conjugate root, but through \(v\) the action propagates to the original roots \(\{\alpha,\beta,\gamma,\delta\}\) as well—this is a pleasant feature of the construction.

In short, the “mapping-like” behavior of \(\rho_i\) makes the name automorphism feel appropriate. The flavor differs from the permutation operators \(\sigma_i\)—even though, in fact, they are the same. We summarize below.

\( \quad\{\rho_1,\rho_2,\ldots,\rho_{24}\} = Gal(F_0(v)/F_0) \cong S_4\)


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