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A001540
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Number of transpositions needed to generate permutations of length n.
(Formerly M1856 N0734)
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2
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0, 2, 8, 36, 184, 1110, 7776, 62216, 559952, 5599530, 61594840, 739138092, 9608795208, 134523132926, 2017846993904, 32285551902480, 548854382342176, 9879378882159186, 187708198761024552, 3754163975220491060
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| R. J. Ord-Smith, Generation of permutation sequences: Part 1, Computer J., 13 (1970), 151-155.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..100
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FORMULA
| E.g.f.: cosh(x)/(1-x) - exp(x).
Recurrence: a(n) = n*a(n-1) + n - (n mod 2).
a(n) = -1 + n!*sum{k=0, [n/2], 1/(2k)!} = -1 + round(n! * cosh(1)).
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EXAMPLE
| a(5)=-1+5!(1+1/2!+1/4!)=-1+120+60+5=184.
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PROG
| (PARI) a(n)=-1+n!*sum(k=0, floor(n/2), 1/(2*k)!)
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CROSSREFS
| a(n) = |A009179(n)|-1 = (n-1)*|A009179(n-1)-A009179(n-2)|.
Sequence in context: A113327 A129148 A081958 * A129044 A052582 A020021
Adjacent sequences: A001537 A001538 A001539 * A001541 A001542 A001543
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KEYWORD
| nonn,easy,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| Edited by Ralf Stephan, Apr 16 2004
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