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 A052747 a(0) = a(1) = a(2) = 0; a(n) = n!/(n-2) for n > 2. 11
 0, 0, 0, 6, 12, 40, 180, 1008, 6720, 51840, 453600, 4435200, 47900160, 566092800, 7264857600, 100590336000, 1494484992000, 23712495206400, 400148356608000, 7155594141696000, 135161222676480000, 2688996956405760000, 56200036388880384000, 1231048416137379840000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS A simple grammar. LINKS Amiram Eldar, Table of n, a(n) for n = 0..450 INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 703. FORMULA E.g.f.: log(-1/(-1+x))*x^2 Recurrence: {a(0)=0, a(1)=0, a(2)=0, a(3)=6, (n+2-n^2)*a(n)+(n-1)*a(n+1)} Sum_{n>=3} 1/a(n) = 3 - e. - Amiram Eldar, Oct 07 2020 MAPLE spec := [S, {B=Cycle(Z), S=Prod(Z, Z, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); MATHEMATICA a[n_] := If[n < 3, 0, n!/(n-2)]; Array[a, 20, 0] (* Amiram Eldar, Oct 07 2020 *) PROG (PARI) a(n)=if(n>2, n!/(n-2), 0) \\ Charles R Greathouse IV, Nov 20 2011 CROSSREFS Equals the square root of the second right hand column of A162990 for n=>3. [Johannes W. Meijer, Jul 21 2009] Sequence in context: A128453 A185616 A307181 * A007121 A152786 A267309 Adjacent sequences:  A052744 A052745 A052746 * A052748 A052749 A052750 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 EXTENSIONS Better description from Olivier Gérard, Jun 13 2001 STATUS approved

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Last modified September 19 08:05 EDT 2021. Contains 347556 sequences. (Running on oeis4.)