a(n) = 7*3^n.

Terms

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