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A133922
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a(n) = number of permutations (p(1),p(2),p(3),...p(n)) of (1,2,3,...n) such that p(k) is coprime to p(n+1-k) for k = all positive integers <=n.
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1, 2, 2, 16, 16, 192, 192, 6912, 4608, 230400, 230400, 11612160, 11612160, 1199923200, 588349440
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| For n = odd integer the middle term of all counted permutations must be 1.
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EXAMPLE
| For n = 6, the permutation (3,2,1,6,4,5) is not counted because p(2)=2 is not coprime to p(5)=4. However, the permutation (3,6,1,4,5,2) is counted because GCD(3,2) = GCD(6,5) = GCD(1,4) = 1.
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CROSSREFS
| Sequence in context: A093114 A016740 A193145 * A088139 A152556 A113123
Adjacent sequences: A133919 A133920 A133921 * A133923 A133924 A133925
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KEYWORD
| more,nonn
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AUTHOR
| Leroy Quet Jan 07 2008
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EXTENSIONS
| a(6)-a(15) from Sean A. Irvine (sairvin(AT)xtra.co.nz), May 17 2010
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