a(n) = Product_{i=0..2} (2^floor((n+i)/3)-1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =9a(6) =27a(7) =63a(8) =147a(9) =343a(10) =735a(11) =1575a(12) =3375a(13) =6975a(14) =14415a(15) =29791a(16) =60543a(17) =123039a(18) =250047a(19) =504063a(20) =1016127a(21) =2048383a(22) =4112895a(23) =8258175a(24) =16581375a(25) =33227775a(26) =66585855a(27) =133432831a(28) =267126783a(29) =534776319