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A052566
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Expansion of e.g.f. (2 + x)/(1 - x^2).
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1
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2, 1, 4, 6, 48, 120, 1440, 5040, 80640, 362880, 7257600, 39916800, 958003200, 6227020800, 174356582400, 1307674368000, 41845579776000, 355687428096000, 12804747411456000, 121645100408832000, 4865804016353280000, 51090942171709440000, 2248001455555215360000
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OFFSET
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0,1
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LINKS
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FORMULA
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Recurrence: {a(1)=1, a(0)=2, (-2 - n^2 - 3*n)*a(n) + a(n+2) = 0}.
Sum((1/2)*(1 + 2*_alpha)*_alpha^(-1-n), _alpha = RootOf(-1 + _Z^2))*n!.
E.g.f.: (x+2)/(1-x^2).
a(n) = 2n! if n is even, n! if odd.
Sum_{n>=0} 1/a(n) = sinh(1) + cosh(1)/2.
Sum_{n>=0} (-1)^(n+1)/a(n) = sinh(1) - cosh(1)/2. (End)
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MAPLE
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spec := [S, {S=Union(Sequence(Z), Sequence(Prod(Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
a:=n->n!+sum((-1)^k*n!, k=0..n): seq(a(n), n=0..19); # Zerinvary Lajos, Mar 25 2008
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MATHEMATICA
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a[n_] := If[OddQ[n], n!, 2*n!]; Array[a, 20, 0] (* Amiram Eldar, Jul 06 2022 *)
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PROG
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(PARI) a(n)=if(n<0, 0, n!*polcoeff((x+2)/(1-x^2)+x*O(x^n), n))
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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STATUS
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approved
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