a(n) = 4^n - n*3^(n-1).

Terms

a(0) =1a(1) =3a(2) =10a(3) =37a(4) =148a(5) =619a(6) =2638a(7) =11281a(8) =48040a(9) =203095a(10) =851746a(11) =3544765a(12) =14651452a(13) =60200131a(14) =246114934a(15) =1001997289a(16) =4065384784a(17) =16448074927a(18) =66394953802a(19) =267516917653