Concatenation of n and n + 4 or {n,n+4}.

Terms

a(0) =15a(1) =26a(2) =37a(3) =48a(4) =59a(5) =610a(6) =711a(7) =812a(8) =913a(9) =1014a(10) =1115a(11) =1216a(12) =1317a(13) =1418a(14) =1519a(15) =1620a(16) =1721a(17) =1822a(18) =1923a(19) =2024a(20) =2125a(21) =2226a(22) =2327a(23) =2428a(24) =2529a(25) =2630a(26) =2731a(27) =2832a(28) =2933a(29) =3034