Expansion of g.f. ((1+x)/(1-3*x))^2.

Terms

a(0) =1a(1) =8a(2) =40a(3) =168a(4) =648a(5) =2376a(6) =8424a(7) =29160a(8) =99144a(9) =332424a(10) =1102248a(11) =3621672a(12) =11809800a(13) =38263752a(14) =123294312a(15) =395392104a(16) =1262703816a(17) =4017693960a(18) =12741829416a(19) =40291730856a(20) =127073920392a(21) =399817944648