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A007999
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a(n) is the number of permutations w of 1,2,...,n such that both w and w^{-1} are alternating.
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3
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1, 1, 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, 487341646372204813
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OFFSET
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0,5
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LINKS
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FORMULA
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G.f.: Sum_{k>=0} E_{2k+1}^2 u^(2k+1)/(2k+1)! + (1-x^2)^(-1/2) Sum_{k>=0} E_{2k}^2 u^(2k)/(2k)!, where E_j is an Euler number (A000111) and u = (1/2)*log((1+x)/(1-x)). - Richard Stanley, Jan 21 2006
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EXAMPLE
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The only alternating permutation of 1,2,3 whose inverse is alternating is 132.
The two alternating permutations of 1,2,3,4 whose inverses are alternating are 1324 and 3412.
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MATHEMATICA
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m = 27;
e[n_] := If[EvenQ[n], Abs[EulerE[n]], Abs[(2^(n+1)(2^(n+1)-1)*BernoulliB[ n+1])/(n+1)]];
u[x_] := Log[(1+x)/(1-x)]/2;
Sum[e[2k+1]^2 u[x]^(2k+1)/(2k+1)!, {k, 0, m}] + (1-x^2)^(-1/2) Sum[e[2k]^2* u[x]^(2k)/(2k)!, {k, 0, m}] + O[x]^m // CoefficientList[#, x]& (* Jean-François Alcover, Feb 24 2019 *)
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CROSSREFS
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For even n > 1, a(n) - a(n-2) = A332344(n).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Two initial terms (thus correcting first term index, and consequent correction of Mathematica code) added by David Bevan, Feb 10 2020
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STATUS
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approved
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