Well-order the rational numbers; take denominators.

Terms

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