In how many ways 20 people be re arrange so that everyone had a different right hand neighbour in new arrangement
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$18795307255050944540$ – Donald Splutterwit Nov 17 '17 at 15:32
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What have you attempted? Have you worked out the first few cases? For small $n$$, using the Inclusion-Exclusion Principle is an option. However, the best approach is to construct a recurrence relation. – N. F. Taussig Nov 18 '17 at 11:18