a(n) = 3^n - 2^n - 2*n - 1, n > 1; a(0)=a(1)=0.

Terms

a(0) =0a(1) =0a(2) =0a(3) =12a(4) =56a(5) =200a(6) =652a(7) =2044a(8) =6288a(9) =19152a(10) =58004a(11) =175076a(12) =527320a(13) =1586104a(14) =4766556a(15) =14316108a(16) =42981152a(17) =129009056a(18) =387158308a(19) =1161737140a(20) =3485735784a(21) =10458256008a(22) =31376865260a(23) =94134790172a(24) =282412759216a(25) =847255054960