Start with 1; for n>1, replace n with the concatenation of its prime factors in increasing order.

Terms

a(0) =1a(1) =2a(2) =3a(3) =22a(4) =5a(5) =23a(6) =7a(7) =222a(8) =33a(9) =25a(10) =11a(11) =223a(12) =13a(13) =27a(14) =35a(15) =2222a(16) =17a(17) =233a(18) =19a(19) =225a(20) =37a(21) =211a(22) =23a(23) =2223a(24) =55a(25) =213a(26) =333a(27) =227a(28) =29a(29) =235