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

Terms

a(0) =-2a(1) =0a(2) =10a(3) =48a(4) =178a(5) =600a(6) =1930a(7) =6048a(8) =18658a(9) =57000a(10) =173050a(11) =523248a(12) =1577938a(13) =4750200a(14) =14283370a(15) =42915648a(16) =128878018a(17) =386896200a(18) =1161212890a(19) =3484687248a(20) =10456158898a(21) =31372671000a(22) =94126401610a(23) =282395982048a(24) =847221500578