A052624 E.g.f. (1+x^2-2x^3+x^4)/(1-x)^2.

1, 2, 8, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800, 479001600, 6227020800, 87178291200, 1307674368000, 20922789888000, 355687428096000, 6402373705728000, 121645100408832000, 2432902008176640000

Offset

0,2

Links

Table of n, a(n) for n=0..19.

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 570

Formula

E.g.f.: (1+x^2-2*x^3+x^4)/(-1+x)^2

Recurrence: {a(0)=1, a(1)=2, a(2)=8, a(3)=24, (-2-n)*a(n)+a(n+1)=0, a(4)=120}

(n+1)!, n>2.

Maple

spec := [S, {S=Union(Prod(Sequence(Z), Sequence(Z)), Prod(Z, Z))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);

Mathematica

With[{nn=20}, CoefficientList[Series[(1+x^2-2x^3+x^4)/(1-x)^2, {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Oct 31 2016 *)

Crossrefs

Sequence in context: A150669 A157005 A123775 * A272590 A361991 A087982

Adjacent sequences: A052621 A052622 A052623 * A052625 A052626 A052627

Keyword

easy,nonn

Author

encyclopedia(AT)pommard.inria.fr, Jan 25 2000

Status

approved