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 A083674 #27 May 06 2022 13:13:51 %S A083674 1,36,45,23220,105111,135460,2492028,5286126,6604795,14308575, %T A083674 45025305,50516326,54742416,99017628,108125865,152486916,386767578, %U A083674 1083567628,1561818105,3169234305,5005551540,5718242211,6125307903,6479715880 %N A083674 Triangular numbers whose sum of divisors is also a triangular number. %C A083674 1, 5286126, 45025305, 54742416, ... are also hexagonal and their sum of divisors too. - _Michel Marcus_, Apr 20 2014 %H A083674 Donovan Johnson, <a href="/A083674/b083674.txt">Table of n, a(n) for n = 1..200</a> %H A083674 Shyam Sunder Gupta, <a href="http://www.shyamsundergupta.com/triangle.htm">Fascinating Triangular Numbers</a>. %H A083674 Douglas E. Iannucci, Florian Luca, <a href="http://dx.doi.org/10.4064/aa129-1-2">Triangular numbers whose sum of divisors is also triangular</a>, Acta Arithmetica 129(2007), 23-40. %e A083674 a(2)=36 because sum of divisors of 36 =1+2+3+4+6+9+12+18+36=91, which is also a triangular number. %t A083674 Select[Accumulate[Range[120000]],OddQ[Sqrt[8*DivisorSigma[1,#]+1]]&] (* _Harvey P. Dale_, Feb 25 2015 *) %o A083674 (PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(sigma(n), 3); \\ _Michel Marcus_, Apr 20 2014 %Y A083674 Cf. A008848 (similar, with squares). %K A083674 nonn %O A083674 1,2 %A A083674 _Shyam Sunder Gupta_, Jun 15 2003