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

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22 Chapter 1<br />

Remark. Condition 2 follows from conditions 1 <strong>and</strong> 3.<br />

58. F<strong>in</strong>d all subgroups <strong>of</strong> the follow<strong>in</strong>g groups: 1) <strong>of</strong> symmetries <strong>of</strong><br />

the equilateral triangle, 2) <strong>of</strong> symmetries <strong>of</strong> the square.<br />

c)<br />

59. F<strong>in</strong>d all subgroups <strong>of</strong> the follow<strong>in</strong>g cyclic groups: a) b)<br />

60. Prove that all subgroups <strong>of</strong> have the form<br />

where divides <strong>and</strong> is a generator <strong>of</strong> the group<br />

61. Prove that all subgroups <strong>of</strong> an <strong>in</strong>f<strong>in</strong>ite cyclic group are <strong>of</strong> the<br />

type where is a generator <strong>and</strong> is an<br />

arbitrary non zero <strong>in</strong>teger number.<br />

62. Prove that an <strong>in</strong>f<strong>in</strong>ite cyclic group has an <strong>in</strong>f<strong>in</strong>ite number <strong>of</strong><br />

subgroups.<br />

63. Prove that the <strong>in</strong>tersection <strong>of</strong> an arbitrary number <strong>of</strong> subgroups 4<br />

<strong>of</strong> a group G is itself a subgroup <strong>of</strong> group G.<br />

EXAMPLE 10. Consider a regular tetrahedron, with vertices marked<br />

with the letters A,B,C, <strong>and</strong> D. If we look at the triangle ABC from<br />

po<strong>in</strong>t the D, then the rotation def<strong>in</strong>ed by the cyclic order <strong>of</strong> po<strong>in</strong>ts A, B, C<br />

may be a clockwise or counterclockwise rotation (see Figure 5). We shall<br />

dist<strong>in</strong>guish these two different orientations <strong>of</strong> the tetrahedron.<br />

FIGURE 5<br />

64. Is the orientation <strong>of</strong> the tetrahedron preserved by the follow<strong>in</strong>g<br />

permutations: (rotation by 120° around the alti-<br />

4 The <strong>in</strong>tersection <strong>of</strong> many sets is the set <strong>of</strong> all elements belong<strong>in</strong>g at the same time<br />

to all the sets.

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