a(n) is the least number not occurring earlier such that neighboring digits sum to 1 or 10.

Terms

a(0) =0a(1) =1a(2) =9a(3) =10a(4) =19a(5) =101a(6) =91a(7) =910a(8) =191a(9) =919a(10) =1010a(11) =1019a(12) =1910a(13) =1919a(14) =10101a(15) =9101a(16) =9191a(17) =91010a(18) =10191a(19) =91019a(20) =19101a(21) =91910a(22) =19191a(23) =91919a(24) =101010a(25) =101019a(26) =101910a(27) =101919a(28) =191010a(29) =191019