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 A052660 E.g.f. (2-2x-x^2)/((1-x)(1-x-x^2)). 0
 2, 2, 6, 24, 144, 1080, 10080, 110880, 1411200, 20321280, 326592000, 5787936000, 112086374400, 2353813862400, 53265935923200, 1291982275584000, 33434618241024000, 919452001628160000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS Table of n, a(n) for n=0..17. INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 607 FORMULA E.g.f.: -(-2+x^2+2*x)/(-1+x)/(-1+x+x^2) Recurrence: {a(1)=2, a(2)=6, a(0)=2, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0} (1+Sum(1/5*(1+2*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2)))*n! a(n) = n!*A001611(n+1). - R. J. Mathar, Nov 27 2011 MAPLE spec := [S, {S=Union(Sequence(Z), Sequence(Union(Z, Prod(Z, Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); CROSSREFS Sequence in context: A301381 A342282 A253093 * A135407 A292831 A076726 Adjacent sequences: A052657 A052658 A052659 * A052661 A052662 A052663 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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Last modified April 23 08:14 EDT 2024. Contains 371905 sequences. (Running on oeis4.)