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 A001471 Number of degree-n permutations of order exactly 3. (Formerly M1833 N0727) 28
 0, 0, 0, 2, 8, 20, 80, 350, 1232, 5768, 31040, 142010, 776600, 4874012, 27027728, 168369110, 1191911840, 7678566800, 53474964992, 418199988338, 3044269834280, 23364756531620, 199008751634000, 1605461415071822 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS a(n) is the number of non-symmetric permutation matrices A of dimension n such that A^2 is the transpose of A. - Torlach Rush, Jul 09 2020 REFERENCES 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). LINKS T. D. Noe, Table of n, a(n) for n = 0..100 L. Moser and M. Wyman, On solutions of x^d = 1 in symmetric groups, Canad. J. Math., 7 (1955), 159-168. FORMULA From Henry Bottomley, Jan 26 2001: (Start) a(n) = a(n-1) + (1 + a(n-3))*(n-1)(n-2). a(n) = Sum_{j=1..floor(n/3)} n!/(j!*(n-3*j)!*(3^j)). a(n) = A001470(n) - 1. (End) E.g.f.: exp(x + x^3/3) - exp(x). MATHEMATICA a[n_] := HypergeometricPFQ[{1/3-n/3, 2/3-n/3, -n/3}, {}, -9] - 1; Table[a[n], {n, 0, 23}] (* Jean-François Alcover, Oct 19 2011 *) PROG (PARI) a(n)=sum(j=1, n\3, n!/(j!*(n-3*j)!*(3^j))) \\ Charles R Greathouse IV, Jun 21 2017 (PARI) first(n)=my(v=vector(n+1)); for(i=3, n, v[i+1]=v[i] + (1+v[i-2])*(i-1)*(i-2)); v \\ Charles R Greathouse IV, Jul 10 2020 (MAGMA) m:=30; R:=PowerSeriesRing(Rationals(), m); b:=Coefficients(R!( Exp(x + x^3/3) )); [Factorial(n-1)*b[n]-1: n in [1..m]]; // G. C. Greubel, May 14 2019 (Sage) m = 30; T = taylor(exp(x + x^3/3) -exp(x), x, 0, m); [factorial(n)*T.coefficient(x, n) for n in (0..m)] # G. C. Greubel, May 14 2019 CROSSREFS Cf. A000085, A001470, A001472, A052501, A053496-A053505, A001189, A001471, A001473, A061121-A061128. Column k=3 of A057731. Sequence in context: A133467 A091004 A005559 * A162585 A000159 A090612 Adjacent sequences:  A001468 A001469 A001470 * A001472 A001473 A001474 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified April 11 09:53 EDT 2021. Contains 342886 sequences. (Running on oeis4.)