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 A276645 #16 Oct 15 2016 08:54:04
%S A276645 0,1,4,5,9,19,148,209,404,652,763,1739,2790,9551,11117,13173,25641,
%T A276645 170003,181927,519900,3253342,4816257,7103608,8269588,64261168,
%U A276645 177847445,294878941,819667577,844965349,2944185248
%N A276645 Numbers n such that A002088(n) is a triangular number.
%C A276645 Corresponding triangular numbers are 0, 1, 6, 10, 28, 120, 6670, 13366, 49770, 129286, 177310, ...
%e A276645 9 is a term because A002088(9) = 1+1+2+2+4+2+6+4+6 = 28 = 1+2+3+4+5+6+7.
%o A276645 (PARI) lista(nn) = {print1(st = 0, ", "); for(n=1, nn, st += eulerphi(n); if (ispolygonal(st, 3), print1(n, ", ")));} \\ _Michel Marcus_, Sep 09 2016
%Y A276645 Cf. A000217, A002088.
%K A276645 nonn,more
%O A276645 1,3
%A A276645 _Altug Alkan_, Sep 09 2016
%E A276645 More terms from _Michel Marcus_, Sep 09 2016
%E A276645 a(25)-a(30) from _Lars Blomberg_, Oct 15 2016