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 A264738 #26 Jan 30 2016 07:02:22 %S A264738 1,5,3,181,88028,21235051740,567915201450407862150, %T A264738 344114043575504570099838908555592732193828, %U A264738 2517587700386740093077985318903210991887701682095104296188806385635425261250603998340 %N A264738 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is triangular (m*(m+1)/2). %H A264738 Jon E. Schoenfield, <a href="/A264738/b264738.txt">Table of n, a(n) for n = 1..12</a> %H A264738 Wikipedia, <a href="https://en.wikipedia.org/wiki/Triangular_number">Triangular_number</a> %o A264738 (PARI) istriangular(n)=ispolygonal(n,3) %o A264738 first(m)=my(v=vector(m),s="1");v[1]=1;for(i=2,m,n=1;while(!istriangular(eval(concat(s,Str(n)))),n++);v[i]=n;s=concat(s,Str(n)));v %Y A264738 Cf. A000217, A069606. %K A264738 nonn,base %O A264738 1,2 %A A264738 _Anders Hellström_, Nov 22 2015 %E A264738 a(6)-a(9) from _Jon E. Schoenfield_, Nov 22 2015