a(n) = 5 - 2^(n - 1) + 3^(n - 1) + n - 2.

Terms

a(0) =4a(1) =6a(2) =11a(3) =26a(4) =73a(5) =220a(6) =675a(7) =2070a(8) =6317a(9) =19184a(10) =58039a(11) =175114a(12) =527361a(13) =1586148a(14) =4766603a(15) =14316158a(16) =42981205a(17) =129009112a(18) =387158367a(19) =1161737202a(20) =3485735849a(21) =10458256076a(22) =31376865331a(23) =94134790246a(24) =282412759293a(25) =847255055040