A052743 E.g.f. ( 1-x-sqrt(1-2*x+x^2-4*x^3) )/(2*x^2).
0, 1, 2, 6, 48, 480, 5040, 65520, 1048320, 18869760, 377395200, 8462361600, 209802700800, 5666588928000, 165900288153600, 5242466541312000, 177718177308672000, 6430828699975680000, 247490157968621568000
Offset: 0
Links
- INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 699
Programs
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Maple
spec := [S,{B=Prod(C,C),S=Union(B,Z,C),C=Prod(S,Z)},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
Formula
D-finite with recurrence: a(1)=1; a(3)=6; a(2)=2; a(4)=48; (-56*n^2-26*n^3-12-46*n-4*n^4)*a(n) +(n^3+7*n^2+16*n+12)*a(n+1) +(-13*n-2*n^2-21)*a(n+2) +(n+5)*a(n+3) =0.