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 A024255 a(0)=0, a(n) = n*E(2n-1) for n >= 1, where E(n) = A000111(n) are the Euler (or up-down) numbers. 3
 0, 1, 4, 48, 1088, 39680, 2122752, 156577792, 15230058496, 1888788086784, 290888851128320, 54466478584365056, 12185086638082228224, 3209979242472703787008, 983522422455215438430208, 346787762817143967622103040, 139423404114002708738732982272 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Number of cyclically alternating permutations of length 2n. Example: a(2)=4 because we have 1324, 1423, 2314, and 2413 (3412 is alternating but not cyclically alternating). REFERENCES G. Kreweras, Les prĂ©ordres totaux compatibles avec un ordre partiel. Math. Sci. Humaines No. 53 (1976), 5-30. LINKS T. D. Noe, Table of n, a(n) for n = 0..100 N. D. Elkies, On the sums Sum((4k+1)^(-n),k,-inf,+inf), arXiv:math/0101168 [math.CA], 2001-2003. N. D. Elkies, On the sums Sum_{k = -infinity .. infinity} (4k+1)^(-n), Amer. Math. Monthly, 110 (No. 7, 2003), 561-573. FORMULA a(n) = 2^(n-1)*(2^n-1)*|B_n|. E.g.f.: tan(x)*x/2 (even part). a(n) = (2*n)!*Pi^(-2*n)*(4^n-1)*Li{2*n}(1) for n > 0. - Peter Luschny, Jun 29 2012 G.f.: Q(0)*x/(1-4*x), where Q(k) = 1 - 16*x^2*(k+2)*(k+1)^3/( 16*x^2*(k+2)*(k+1)^3 - (1 - 8*x*k^2 - 12*x*k -4*x)*(1 - 8*x*k^2 - 28*x*k -24*x)/Q(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Oct 23 2013 MAPLE a := n -> (-1)^n*2^(2*n-1)*(1-2^(2*n))*bernoulli(2*n); # Peter Luschny, Jun 08 2009 MATHEMATICA nn = 30; t = Range[0, nn]! CoefficientList[Series[Tan[x]*x/2, {x, 0, nn}], x]; Take[t, {1, nn, 2}] CROSSREFS Cf. A009752. Sequence in context: A138448 A071221 A198038 * A211045 A296838 A211049 Adjacent sequences:  A024252 A024253 A024254 * A024256 A024257 A024258 KEYWORD nonn AUTHOR EXTENSIONS Edited by Emeric Deutsch, Jul 01 2009 STATUS approved

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Last modified October 20 17:33 EDT 2018. Contains 316393 sequences. (Running on oeis4.)