a(n) = 3/4*(3^(n + 1) - 2*n - 4*n^2 - 3).

Terms

a(0) =0a(1) =0a(2) =3a(3) =27a(4) =126a(5) =462a(6) =1521a(7) =4761a(8) =14556a(9) =44028a(10) =132543a(11) =398199a(12) =1195290a(13) =3586698a(14) =10761069a(15) =32284341a(16) =96854328a(17) =290564472a(18) =871695099a(19) =2615087187a(20) =7845263670a(21) =23535793350a(22) =70607382633a(23) =211822150737a(24) =635466455316