Concatenation of n and n+7.

Terms

a(0) =18a(1) =29a(2) =310a(3) =411a(4) =512a(5) =613a(6) =714a(7) =815a(8) =916a(9) =1017a(10) =1118a(11) =1219a(12) =1320a(13) =1421a(14) =1522a(15) =1623a(16) =1724a(17) =1825a(18) =1926a(19) =2027a(20) =2128a(21) =2229a(22) =2330a(23) =2431a(24) =2532a(25) =2633a(26) =2734a(27) =2835a(28) =2936a(29) =3037