a(n) = 3^(n-1) - ceiling(n^n/n!).

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =54a(5) =178a(6) =565a(7) =1770a(8) =5493a(9) =16927a(10) =51901a(11) =158533a(12) =482802a(13) =1466859a(14) =4448104a(15) =13467249a(16) =40720970a(17) =122994566a(18) =371156622a(19) =1119161662a(20) =3372427789a(21) =10156591942a(22) =30573367574a(23) =91993546765a(24) =276703494365a(25) =832023918335