Concatenation of n and n + 3.

Terms

a(0) =14a(1) =25a(2) =36a(3) =47a(4) =58a(5) =69a(6) =710a(7) =811a(8) =912a(9) =1013a(10) =1114a(11) =1215a(12) =1316a(13) =1417a(14) =1518a(15) =1619a(16) =1720a(17) =1821a(18) =1922a(19) =2023a(20) =2124a(21) =2225a(22) =2326a(23) =2427a(24) =2528a(25) =2629a(26) =2730a(27) =2831a(28) =2932a(29) =3033