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 A209310 #16 Aug 12 2023 04:37:38
%S A209310 6,4186,32131,52975,78210,111628,237016,247456,584821,750925,1464616,
%T A209310 3649051,5791906,11297881,16082956,24650731,27243271,38618866,
%U A209310 46585378,51546781,56026405,76923406,89880528,96070591,126906346,164629585,201854278,228733966
%N A209310 Triangular numbers whose sum of triangular divisors is also triangular and greater than 1.
%H A209310 Donovan Johnson, <a href="/A209310/b209310.txt">Table of n, a(n) for n = 1..1000</a>
%e A209310 4186 is in sequence because it is triangular (4186 = 91*92/2) and the sum of its triangular divisors, 4186+91+1 = 4278 is also triangular (4278 = 92*93/2).
%t A209310 triQ[n_] := n > 1 && IntegerQ[Sqrt[8*n+1]]; q[n_] := triQ[1 + DivisorSum[n, #&, triQ[#] &]]; Select[Accumulate[Range[22000]], q] (* _Amiram Eldar_, Aug 12 2023 *)
%o A209310 (PARI) istriangular(n)=issquare(8*n+1)
%o A209310 {t=0; for(n=1, 10^8, if(istriangular(n), k=sumdiv(n, d, istriangular(d)*d) ;if(istriangular(k)&&k>>1,t+=1;write("b209310.txt",t," ",n))))}
%Y A209310 Subsequence of A209309.
%Y A209310 Cf. A000217, A185027, A035316.
%K A209310 nonn
%O A209310 1,1
%A A209310 _Antonio Roldán_, Jan 18 2013