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

Terms

a(0) =3a(1) =3a(2) =9a(3) =9a(4) =33a(5) =27a(6) =99a(7) =99a(8) =297a(9) =297a(10) =945a(11) =891a(12) =2835a(13) =2835a(14) =8505a(15) =8505a(16) =26001a(17) =25515a(18) =78003a(19) =78003a(20) =234009a(21) =234009a(22) =706401a(23) =702027a(24) =2119203a(25) =2119203a(26) =6357609a(27) =6357609a(28) =19112193a(29) =19072827