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

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142 Problems <strong>of</strong> Chapter 1<br />

i.e.‚ (Recall that <strong>in</strong> the product the permutation is carried<br />

out first‚ <strong>and</strong> later.)<br />

174. Suppose that the element is sent to to<br />

etc.. Let<br />

be the first return<strong>in</strong>g element. If we suppose that where<br />

then we obta<strong>in</strong> that two dist<strong>in</strong>ct elements‚ <strong>and</strong><br />

are sent by the permutation to the same element‚ which is a contradiction.<br />

It follows that <strong>and</strong> one obta<strong>in</strong>s a cycle. Start<strong>in</strong>g with an arbitrary<br />

element‚ not belong<strong>in</strong>g to this cycle‚ one constructs the second cycle‚ etc..<br />

It is easy to see that every permutation is the product <strong>of</strong> <strong>in</strong>dependent<br />

cycles.<br />

Suppose now a given permutation be the product <strong>of</strong> <strong>in</strong>dependent cycles.<br />

If one <strong>of</strong> the cycles sends the element to <strong>and</strong> the elements<br />

<strong>and</strong> do not appear <strong>in</strong> other cycles‚ then all products <strong>of</strong> cycles send<br />

to Hence the element which follows <strong>in</strong> the cycle conta<strong>in</strong><strong>in</strong>g<br />

is uniquely def<strong>in</strong>ed by the given permutation. Therefore all cycles<br />

are uniquely def<strong>in</strong>ed. Note that if the cycles are not <strong>in</strong>dependent the<br />

decomposition <strong>in</strong>to cycles may not be unique. For example‚<br />

175. H<strong>in</strong>t. Verify this equality<br />

176. H<strong>in</strong>t. Let Verify the equality<br />

177. The pairs correspond<strong>in</strong>g to <strong>in</strong>versions are (3‚2)‚ (3‚1)‚ (2‚1)‚<br />

(5‚4)‚ (5‚1)‚ (4‚1). Answer. 6.<br />

178. If the numbers <strong>and</strong> are <strong>in</strong>terchanged <strong>and</strong> are<br />

the numbers between <strong>and</strong> the property <strong>of</strong> be<strong>in</strong>g or not be<strong>in</strong>g an<br />

<strong>in</strong>version changes <strong>in</strong>to the opposite for the pairs <strong>of</strong> the follow<strong>in</strong>g numbers:<br />

where i.e.‚ for pairs. S<strong>in</strong>ce the<br />

number is odd the parity <strong>of</strong> the number <strong>of</strong> <strong>in</strong>versions does change.<br />

179. Answer. The permutation is even (6 <strong>in</strong>versions).

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