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 A264777 #32 Dec 20 2015 13:46:32 %S A264777 1,8,9,5296,9701946,3254409476919,214948107829163565920446430, %T A264777 31021083401526077498957998513210666182754941982955902, %U A264777 81969637737594122848099953820072074027662186843090573669278105078583525219219292274950391247747733666835448 %N A264777 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is a heptagonal number (A000566). %C A264777 a(20) has 219815 decimal digits. - _Chai Wah Wu_, Nov 25 2015 %H A264777 Chai Wah Wu, <a href="/A264777/b264777.txt">Table of n, a(n) for n = 1..12</a> %H A264777 Wikipedia, <a href="https://en.wikipedia.org/wiki/Heptagonal_number">Heptagonal number</a> %e A264777 1, 18, 189, 1895296 are heptagonal. %o A264777 (PARI) isheptagonal(n)=ispolygonal(n,7) %o A264777 first(m)=my(v=vector(m),s="");s="1";v[1]=1;for(i=2,m,n=1;while(!isheptagonal(eval(concat(s,Str(n)))),n++);v[i]=n;s=concat(s,Str(n)));v %Y A264777 Cf. A000566, A264738. %K A264777 nonn,base %O A264777 1,2 %A A264777 _Anders Hellström_, Nov 24 2015 %E A264777 a(6)-a(8) from _Jon E. Schoenfield_, Nov 24 2015 %E A264777 a(9) from _Chai Wah Wu_, Nov 25 2015