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Abel's theorem in problems and solutions - School of Mathematics

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32 Chapter 1<br />

120. F<strong>in</strong>d the commutant <strong>in</strong> the group <strong>of</strong> symmetries <strong>of</strong> the tetrahedron.<br />

121. Prove that if a normal subgroup <strong>of</strong> the group <strong>of</strong> rotations or <strong>of</strong><br />

the group <strong>of</strong> symmetries <strong>of</strong> the tetrahedron conta<strong>in</strong>s just a sole rotation<br />

around one axis through a vertex, then it conta<strong>in</strong>s all rotations <strong>of</strong> the<br />

tetrahedron.<br />

122. F<strong>in</strong>d the commutant <strong>in</strong> the group <strong>of</strong> symmetries <strong>of</strong> the tetrahedron.<br />

Consider two groups: the group <strong>of</strong> rotations <strong>of</strong> the cube <strong>and</strong> the group<br />

<strong>of</strong> rotations <strong>of</strong> the regular octahedron.<br />

FIGURE 7<br />

123. How many elements are conta<strong>in</strong>ed <strong>in</strong> these groups? Calculate<br />

the elements <strong>of</strong> the group <strong>of</strong> rotations <strong>of</strong> the cube.<br />

124. Prove that the groups <strong>of</strong> rotations <strong>of</strong> the cube <strong>and</strong> <strong>of</strong> the octahedron<br />

are isomorphic.<br />

125. In how many different ways is it possible to colour the surface<br />

<strong>of</strong> a cube with 6 colours (a different colour for each face) if one considers<br />

two coloured cubes as different if they do not co<strong>in</strong>cide even after some<br />

rotation? The same question for a box <strong>of</strong> matches.<br />

126. Which group amongst those you know is isomorphic to the group<br />

<strong>of</strong> rotations <strong>of</strong> a box <strong>of</strong> matches?<br />

To calculate the commutant <strong>of</strong> the group <strong>of</strong> rotations <strong>of</strong> the cube we<br />

<strong>in</strong>scribe <strong>in</strong> the cube a tetrahedron (see Figure 8).<br />

Jo<strong>in</strong><strong>in</strong>g the rema<strong>in</strong><strong>in</strong>g vertices B, D, <strong>and</strong> one obta<strong>in</strong>s a second<br />

tetrahedron. Any rotation <strong>of</strong> the cube either sends each tetrahedron onto<br />

itself or exchanges the tetrahedra with each other.

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