a(n) = (7*3^n - 4*0^n)/3.

Terms

a(0) =1a(1) =7a(2) =21a(3) =63a(4) =189a(5) =567a(6) =1701a(7) =5103a(8) =15309a(9) =45927a(10) =137781a(11) =413343a(12) =1240029a(13) =3720087a(14) =11160261a(15) =33480783a(16) =100442349a(17) =301327047a(18) =903981141a(19) =2711943423a(20) =8135830269a(21) =24407490807a(22) =73222472421a(23) =219667417263a(24) =659002251789