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Search: id:A072948
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| A072948 |
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Number of permutations p of (1,2,3,...,2n) such that sum(k=1,2n,abs(k-p(k)))=2n. |
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2
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OFFSET
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1,2
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FORMULA
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This is impossible if the number of symbols is odd.
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PROGRAM
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(PARI) a(n)=sum(k=1, n!, if(sum(i=1, n, abs(i-component(numtoperm(n, k), i)))-n, 0, 1))
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CROSSREFS
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Cf. A072949.
Sequence in context: A081894 A128597 A067318 this_sequence A000823 A036944 A068640
Adjacent sequences: A072945 A072946 A072947 this_sequence A072949 A072950 A072951
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KEYWORD
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more,nonn
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AUTHOR
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Benoit Cloitre (benoit7848c(AT)orange.fr), Aug 20 2002
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EXTENSIONS
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One more term from Michel ten Voorde (seqfan(AT)tenvoorde.org) Jun 13 2003
17222 from Ryan Propper (rpropper(AT)stanford.edu), Mar 26 2007
2 more terms Sean A. Irvine (sairvin(AT)xtra.co.nz), Sep 22 2009
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