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 A264804 #17 Jan 26 2024 12:47:40 %S A264804 1,1,526,64095,21420730041,4528059468080555555556, %T A264804 3834345160635370971474665069772601398563211, %U A264804 100751687713984558500838936986634939491022212000570658953744730444103042117925197608458 %N A264804 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 11-gonal: (9n^2 - 7n)/2. %H A264804 Chai Wah Wu, <a href="/A264804/b264804.txt">Table of n, a(n) for n = 1..11</a> %H A264804 Wikipedia, <a href="https://en.wikipedia.org/wiki/Polygonal_number">Polygonal number</a> %o A264804 (PARI) hendecagonal(n)=ispolygonal(n,11) %o A264804 first(m)=my(v=vector(m),s="");s="1";print1(1, ", ");for(i=2,m,n=1;while(!hendecagonal(eval(concat(s,Str(n)))),n++);print1(n, ", ");s=concat(s,Str(n))) %Y A264804 Cf. A051671, A051682 (11-gonal numbers), A061109, A061110, A261696, A264733, A264738, A264776, A264777, A264842, A264848, A264849. %K A264804 nonn,base %O A264804 1,3 %A A264804 _Anders Hellström_, Nov 25 2015 %E A264804 a(5)-a(8) from _Chai Wah Wu_, Mar 16 2018