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 A343426 #5 Apr 15 2021 21:45:14
%S A343426 276,741,17766,30876,42778,43071,44850,54946,73920,99681,163306,
%T A343426 184528,254541,310866,446040,524800,963966,1006071,1046181,1160526,
%U A343426 1258491,1873080,1929630,2793066,3034416,3108771,3121251,3454506,3635556,4305645,4317391,4435731,4831386,4859403
%N A343426 Triangular numbers T(i) that can be expressed as the sum of 2 positive triangular numbers, T(j)+T(k), and for which i+j+k is a triangular number, where T is A000217.
%H A343426 K. R. S. Sastry, <a href="https://www.jstor.org/stable/2687090">Problem 518</a>, The College Mathematics Journal, p. 64, Vol. 25, No. 1, Jan., 1994; Solution, <a href="https://www.jstor.org/stable/2687297">Pythagorean Triples of Triangular Numbers</a>, p. 69, Vol. 26, No. 1, Jan., 1995.
%e A343426 276 = T(23) = 105 + 171 = T(14) + T(18) and 23+14+18 = 55 = T(10), so 276 is a term.
%o A343426 (PARI) lista(nn) = {for (n=1, nn, my(t = n*(n+1)/2, kk); for (k=1, n-1, if (ispolygonal(t - k*(k+1)/2, 3, &kk), if (ispolygonal(n+k+kk, 3), print1(t, ", "); break;););););}
%Y A343426 Cf. A000217. Subsequence of A089982.
%K A343426 nonn
%O A343426 1,1
%A A343426 _Michel Marcus_, Apr 15 2021