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

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =66a(5) =212a(6) =666a(7) =2060a(8) =6306a(9) =19172a(10) =58026a(11) =175100a(12) =527346a(13) =1586132a(14) =4766586a(15) =14316140a(16) =42981186a(17) =129009092a(18) =387158346a(19) =1161737180a(20) =3485735826a(21) =10458256052a(22) =31376865306a(23) =94134790220a(24) =282412759266a(25) =847255055012