This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A248038 #18 Jan 14 2019 17:30:19 %S A248038 33,66,99,1212,1515,1818,2121,2424,2727,3030,3333,3636,3939,4242,4545, %T A248038 4848,5151,5454,5757,6060,6363,6666,6969,7272,7575,7878,8181,8484, %U A248038 8787,9090,9393,9696,9999,102102,105105,108108,111111,114114,117117,120120,123123 %N A248038 3n concatenated with itself. %t A248038 #*10^IntegerLength[#]+#&/@(3*Range[50]) (* _Harvey P. Dale_, Jan 14 2019 *) %K A248038 nonn,base,easy %O A248038 1,1 %A A248038 _Larry Guindine_, Sep 30 2014 %E A248038 Definition edited by _N. J. A. Sloane_, Sep 30 2014 %E A248038 More terms from _Alois P. Heinz_, Sep 30 2014