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 A264842 #23 Jan 23 2017 13:08:10 %S A264842 1,3,36,54765,123152388,374848814886363636, %T A264842 85794018663817263665487289502938826, %U A264842 107072047880615405294526336549204869795454545454545454545454545454545466 %N A264842 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 13-gonal: (11n^2 - 9n)/2. %H A264842 Jon E. Schoenfield, <a href="/A264842/b264842.txt">Table of n, a(n) for n = 1..11</a> %H A264842 Wikipedia, <a href="https://en.wikipedia.org/wiki/Polygonal_number">Polygonal number</a> %e A264842 1, 13, 1336, 133654765 are 13-gonal. %o A264842 (PARI) tridecagonal(n)=ispolygonal(n, 13) %o A264842 first(m)=my(s=""); s="1"; print1(1, ", "); for(i=2, m, n=1; while(!tridecagonal(eval(concat(s, Str(n)))), n++); print1(n, ", "); s=concat(s, Str(n))) %Y A264842 Cf. A051671, A051865 (13-gonal numbers), A061109, A061110, A261696, A264733, A264738, A264776, A264777, A264848, A264849, A264804. %K A264842 nonn,base %O A264842 1,2 %A A264842 _Anders Hellström_, Nov 26 2015 %E A264842 More terms from _Jon E. Schoenfield_, Nov 27 2015