Odd digits in decreasing order.

Terms

a(0) =1a(1) =3a(2) =5a(3) =7a(4) =9a(5) =31a(6) =51a(7) =53a(8) =71a(9) =73a(10) =75a(11) =91a(12) =93a(13) =95a(14) =97a(15) =531a(16) =731a(17) =751a(18) =753a(19) =931a(20) =951a(21) =953a(22) =971a(23) =973a(24) =975a(25) =7531a(26) =9531a(27) =9731a(28) =9751a(29) =9753