Concatenation of n and n + 9 or {n,n+9}.

Terms

a(0) =110a(1) =211a(2) =312a(3) =413a(4) =514a(5) =615a(6) =716a(7) =817a(8) =918a(9) =1019a(10) =1120a(11) =1221a(12) =1322a(13) =1423a(14) =1524a(15) =1625a(16) =1726a(17) =1827a(18) =1928a(19) =2029a(20) =2130a(21) =2231a(22) =2332a(23) =2433a(24) =2534a(25) =2635a(26) =2736a(27) =2837a(28) =2938a(29) =3039