Elevator buttons in U.S.A.: Positive integers except 13.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =5a(5) =6a(6) =7a(7) =8a(8) =9a(9) =10a(10) =11a(11) =12a(12) =14a(13) =15a(14) =16a(15) =17a(16) =18a(17) =19a(18) =20a(19) =21a(20) =22a(21) =23a(22) =24a(23) =25a(24) =26a(25) =27a(26) =28a(27) =29a(28) =30a(29) =31