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 A226863 #12 Jul 29 2016 14:16:27
%S A226863 3,10,28,55,91,136,253,325,406,496,595,703,946,1081,1225,1378,1711,
%T A226863 1891,2278,2485,2701,2926,3403,3655,3916,4465,4753,5356,5671,7021,
%U A226863 7381,8128,8515,8911,9316,10153,10585,11026,11476,12403,13366,13861,14365,14878,15931
%N A226863 Triangular numbers not divisible by lesser triangular numbers > 1.
%C A226863 We may coin them "prime triangulars". Certainly there are an infinity of them.
%H A226863 Zak Seidov, <a href="/A226863/b226863.txt">Table of n, a(n) for n = 1..10000</a>
%F A226863 a(10000) = 1085943106 = A000217(46603).
%t A226863 nn = 200; tri = Table[n (n + 1)/2, {n, nn}]; t = {}; Do[i = 2; While[i < n && Mod[tri[[n]], tri[[i]]] > 0, i++]; If[i == n, AppendTo[t, tri[[n]]]], {n, nn}]; t (* _T. D. Noe_, Jun 20 2013 *)
%o A226863 (PARI) is(n)=fordiv(n, d, if(ispolygonal(d,3) && d>1 && d<n, return(0))); ispolygonal(n,3) && n>1 \\ _Charles R Greathouse IV_, Jul 29 2016
%Y A226863 Cf. A000217, A137281 (the indices of these triangular numbers).
%K A226863 nonn
%O A226863 1,1
%A A226863 _Zak Seidov_, Jun 20 2013