a(n) = 3^floor((n+1)/2).

Terms

a(0) =1a(1) =3a(2) =3a(3) =9a(4) =9a(5) =27a(6) =27a(7) =81a(8) =81a(9) =243a(10) =243a(11) =729a(12) =729a(13) =2187a(14) =2187a(15) =6561a(16) =6561a(17) =19683a(18) =19683a(19) =59049a(20) =59049a(21) =177147a(22) =177147a(23) =531441a(24) =531441a(25) =1594323a(26) =1594323a(27) =4782969a(28) =4782969a(29) =14348907