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 A052503 Number of permutations sigma of [2n] without fixed points such that sigma^4 = Id. 4
 1, 1, 9, 105, 2625, 76545, 3440745, 176080905, 12034447425, 922995698625, 87505195602825, 9203114782686825, 1141501848477415425, 155540530213013570625, 24232951756530007115625, 4112826185329479728735625, 781060320618828163499210625 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS G. C. Greubel, Table of n, a(n) for n = 0..280 INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 28 FORMULA a(n) = (2n)! * [x^(2n)] exp(x^2/2 + x^4/4). D-finite with recurrence a(n) +(-2*n+1)*a(n-1) -2*(n-1)*(2*n-1)*(2*n-3)*a(n-2)=0, with a(0)=1, a(1)=1, a(2)=9. - Corrected by R. J. Mathar, Feb 20 2020 to skip zeros. a(n) = 2^n*Gamma(n+1/2)*A047974(n)/Pi^(1/2). - Mark van Hoeij, Oct 30 2011 MAPLE spec := [S, {S=Set(Union(Cycle(Z, card=2), Cycle(Z, card=4)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); MATHEMATICA With[{nmax = 40}, CoefficientList[Series[Exp[x^2*(2 + x^2)/4], {x, 0, nmax}], x]*(Range[0, nmax])!][[1 ;; -1 ;; 2]] (* G. C. Greubel, May 14 2019 *) PROG (PARI) x='x+O('x^40); v=Vec(serlaplace( exp(x^2/2 + x^4/4) )); vector(#v\2, n, v[2*n-1]) \\ G. C. Greubel, May 14 2019 (Magma) m:=40; R:=PowerSeriesRing(Rationals(), m); b:=Coefficients(R!( Exp(x^2/2 + x^4/4) )); [Factorial(2*n-2)*b[2*n-1]: n in [1..Floor((m-2)/2)]]; // G. C. Greubel, May 14 2019 (Sage) m = 40; T = taylor(exp(x^2/2 + x^4/4), x, 0, 2*m+2); [factorial(2*n)*T.coefficient(x, 2*n) for n in (0..m)] # G. C. Greubel, May 14 2019 CROSSREFS Cf. A001472, A261317, A261381. Bisection of column k=4 of A261430. Sequence in context: A231646 A110698 A012485 * A261428 A122569 A309652 Adjacent sequences: A052500 A052501 A052502 * A052504 A052505 A052506 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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Last modified December 8 08:16 EST 2022. Contains 358691 sequences. (Running on oeis4.)