a(n) is the concatenation of n and 7n.

Terms

a(0) =17a(1) =214a(2) =321a(3) =428a(4) =535a(5) =642a(6) =749a(7) =856a(8) =963a(9) =1070a(10) =1177a(11) =1284a(12) =1391a(13) =1498a(14) =15105a(15) =16112a(16) =17119a(17) =18126a(18) =19133a(19) =20140a(20) =21147a(21) =22154a(22) =23161a(23) =24168a(24) =25175a(25) =26182a(26) =27189a(27) =28196a(28) =29203a(29) =30210