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 A052566 E.g.f.: (2 + x)/(1 - x^2). 1
 2, 1, 4, 6, 48, 120, 1440, 5040, 80640, 362880, 7257600, 39916800, 958003200, 6227020800, 174356582400, 1307674368000, 41845579776000, 355687428096000, 12804747411456000, 121645100408832000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 508 FORMULA 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. MAPLE 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 PROG (PARI) a(n)=if(n<0, 0, n!*polcoeff((x+2)/(1-x^2)+x*O(x^n), n)) CROSSREFS Sequence in context: A019142 A268573 A145858 * A234946 A223092 A071948 Adjacent sequences:  A052563 A052564 A052565 * A052567 A052568 A052569 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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Last modified June 17 21:47 EDT 2019. Contains 324200 sequences. (Running on oeis4.)