A052638 - OEIS

%I #14 Apr 18 2017 07:03:58

%S 0,0,2,18,96,960,11520,161280,2580480,46448640,928972800,20437401600,

%T 490497638400,12752938598400,357082280755200,10712468422656000,

%U 342798989524992000,11655165643849728000,419585963178590208000

%N E.g.f. x^2*(1+x-2x^2)/(1-2x).

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=584">Encyclopedia of Combinatorial Structures 584</a>

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

%F Recurrence: {a(1)=0, a(0)=0, a(2)=2, (-2*n-2)*a(n)+a(n+1)=0, a(3)=18, a(4)=96}

%F 1/4*2^n*n!

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

%t With[{nn=20},CoefficientList[Series[x^2 (1+x-2x^2)/(1-2x),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Feb 29 2012 *)

%K easy,nonn

%O 0,3

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