a(n) = 3^n - 6*binomial(n,4) - 6*binomial(n,3) - 4*binomial(n,2) - 2*n - 1.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =18a(5) =102a(6) =446a(7) =1668a(8) =5676a(9) =18260a(10) =56868a(11) =173934a(12) =526862a(13) =1587978a(14) =4774386a(15) =14337536a(16) =43031928a(17) =129121224a(18) =387396584a(19) =1162231674a(20) =3486747690a(21) =10460308430a(22) =31381005510a(23) =94143114012a(24) =282429459428a(25) =847288518492