a(n) = 3^n - 3*2^(n-2).

Terms

a(0) =6a(1) =21a(2) =69a(3) =219a(4) =681a(5) =2091a(6) =6369a(7) =19299a(8) =58281a(9) =175611a(10) =528369a(11) =1588179a(12) =4770681a(13) =14324331a(14) =42997569a(15) =129041859a(16) =387223881a(17) =1161868251a(18) =3485997969a(19) =10458780339a(20) =31377913881a(21) =94136887371a(22) =282416953569a(23) =847263443619