A052643 E.g.f. (1+x-x^2)^2/(1-x)^2.

1, 4, 12, 36, 168, 960, 6480, 50400, 443520, 4354560, 47174400, 558835200, 7185024000, 99632332800, 1482030950400, 23538138624000, 397533007872000, 7113748561920000, 134449847820288000, 2676192208994304000

Offset

0,2

Links

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

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 589

Formula

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

Recurrence: {a(0)=1, a(1)=4, a(3)=36, a(4)=168, (-n^2-5*n-4)*a(n)+(n+3)*a(n+1)=0, a(2)=12}

(n+3)*n!, n>2.

Maple

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

Mathematica

With[{nn=20}, CoefficientList[Series[(1+x-x^2)^2/(1-x)^2, {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Mar 06 2016 *)

Crossrefs

Sequence in context: A252697 A056383 A293857 * A140123 A164853 A076124

Adjacent sequences: A052640 A052641 A052642 * A052644 A052645 A052646

Keyword

easy,nonn

Author

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

Status

approved