a(n) = 3*a(n-1) for n>2; a(0)=1, a(1)=3, a(2)=7.

Terms

a(0) =1a(1) =3a(2) =7a(3) =21a(4) =63a(5) =189a(6) =567a(7) =1701a(8) =5103a(9) =15309a(10) =45927a(11) =137781a(12) =413343a(13) =1240029a(14) =3720087a(15) =11160261a(16) =33480783a(17) =100442349a(18) =301327047a(19) =903981141a(20) =2711943423a(21) =8135830269a(22) =24407490807a(23) =73222472421a(24) =219667417263a(25) =659002251789