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

Terms

a(0) =1a(1) =7a(2) =3a(3) =21a(4) =9a(5) =63a(6) =27a(7) =189a(8) =81a(9) =567a(10) =243a(11) =1701a(12) =729a(13) =5103a(14) =2187a(15) =15309a(16) =6561a(17) =45927a(18) =19683a(19) =137781a(20) =59049a(21) =413343a(22) =177147a(23) =1240029a(24) =531441a(25) =3720087a(26) =1594323a(27) =11160261a(28) =4782969a(29) =33480783