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 A034866 a(n) = n!*(n-4)/2, n > 4, and a(4) = 4. 1
 4, 60, 720, 7560, 80640, 907200, 10886400, 139708800, 1916006400, 28021593600, 435891456000, 7192209024000, 125536739328000, 2311968282624000, 44816615940096000, 912338253066240000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,1 LINKS G. C. Greubel, Table of n, a(n) for n = 4..445 J. Riordan, Enumeration of trees by height and diameter, IBM J. Res. Dev. 4 (1960), 473-478. FORMULA a(n) = A034865(n), n > 4. - R. J. Mathar, Oct 20 2008 (-n+5)*a(n) + n*(n-4)*a(n-1) = 0. - R. J. Mathar, Apr 03 2017 E.g.f.: x^4*(1 + x + x^2)/(6*(1 - x)^2). - G. C. Greubel, Feb 16 2018 MAPLE [4, seq(factorial(n)*(n-4)/2, n=5..20)]; # Muniru A Asiru, Feb 17 2018 MATHEMATICA Join[{4}, Table[n!*(n-4)/2, {n, 5, 30}]] (* or *) Drop[With[{nn = 30}, CoefficientList[Series[x^4*(1 + x + x^2)/(6*(1 - x)^2), {x, 0, nn}], x]*Range[0, nn]!], 4]  (* G. C. Greubel, Feb 16 2018 *) PROG (PARI) x='x+O('x^30); Vec(serlaplace(x^4*(1+x+x^2)/(6*(1-x)^2))) \\ G. C. Greubel, Feb 16 2018 (MAGMA) [4] cat [Factorial(n)*(n-4)/2: n in [5..30]]; // G. C. Greubel, Feb 16 2018 (GAP) A034866:=Concatenation([4], List([5..20], n->Factorial(n)*(n-4)/2)); # Muniru A Asiru, Feb 17 2018 CROSSREFS Sequence in context: A002060 A247739 A007220 * A055315 A013482 A123480 Adjacent sequences:  A034863 A034864 A034865 * A034867 A034868 A034869 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified August 7 23:50 EDT 2022. Contains 355995 sequences. (Running on oeis4.)