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Search: id:A007999
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| A007999 |
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a(n)=number of permutations w of 1,2,...,n such that w and w^{-1} are alternating. |
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+0
1
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| 1, 1, 2, 3, 8, 19, 64, 213, 880, 3717, 18288, 92935, 531440, 3147495, 20525168, 138638825, 1015694832, 7700244745, 62623847536, 526317901451, 4705365925872, 43407723925499, 423149546210416, 4250149857500861, 44868038386273776
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OFFSET
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0,3
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REFERENCES
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Foulkes, H. O.; Tangent and secant numbers and representations of symmetric groups. Discrete Math. 15 (1976), no. 4, 311-324.
R. P. Stanley, Alternating permutations and symmetric functions, in preparation.
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FORMULA
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sum_{n=0..infinity} a(n)x^n = sum_{k=0..infinity} E_{2k+1}^2 u^{2k+1}/(2k+1)! + (1-x^2)^{-1/2} sum_{k=0..infinity} E_{2k}^2 u^{2k}/(2k)!, where E_j is an Euler number and u = (1/2)log((1+x)/(1-x)). - R. P. Stanley (rstan(AT)math.mit.edu), Jan 21 2006
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CROSSREFS
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Adjacent sequences: A007996 A007997 A007998 this_sequence A008000 A008001 A008002
Sequence in context: A041281 A078343 A077269 this_sequence A006609 A005663 A112834
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KEYWORD
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nonn
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AUTHOR
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poirier(AT)lacim.uqam.ca, Simon Plouffe (plouffe(AT)math.uqam.ca)
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EXTENSIONS
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More terms from Vladeta Jovovic (vladeta(AT)Eunet.yu), May 15 2007
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