Concatenation of n and n + 6 or {n,n+6}.

Terms

a(0) =17a(1) =28a(2) =39a(3) =410a(4) =511a(5) =612a(6) =713a(7) =814a(8) =915a(9) =1016a(10) =1117a(11) =1218a(12) =1319a(13) =1420a(14) =1521a(15) =1622a(16) =1723a(17) =1824a(18) =1925a(19) =2026a(20) =2127a(21) =2228a(22) =2329a(23) =2430a(24) =2531a(25) =2632a(26) =2733a(27) =2834a(28) =2935a(29) =3036