a(n) = n concatenated with n + 1.

Terms

a(0) =12a(1) =23a(2) =34a(3) =45a(4) =56a(5) =67a(6) =78a(7) =89a(8) =910a(9) =1011a(10) =1112a(11) =1213a(12) =1314a(13) =1415a(14) =1516a(15) =1617a(16) =1718a(17) =1819a(18) =1920a(19) =2021a(20) =2122a(21) =2223a(22) =2324a(23) =2425a(24) =2526a(25) =2627a(26) =2728a(27) =2829a(28) =2930a(29) =3031