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 A213189 #9 Feb 28 2013 17:25:39
%S A213189 6,36,91,120,210,253,300,378,528,630,1176,2016,2346,3003,3240,3828,
%T A213189 4560,4656,4950,5460,6105,6903,7140,7260,8778,10296,11628,13530,14028,
%U A213189 14196,15400,17766,19110,23220,23436,24310,25200,26796,32640,34980,41616,43365,44253,52326,55278
%N A213189 Triangular numbers which are leg of a Pythagorean triple with hypotenuse in A213188.
%e A213189 Triangular 91 and triangular 325 form a Pythagorean triple {325, 91, 312}.
%o A213189 (PARI) {for(i=1,10^3,k=i+1;v=1;a=i*(i+1)/2;while(k<i*i&&v,b=k*(k+1)/2;if(issquare(b*b-a*a),v=0;print1(a,", "));k+=1))}
%Y A213189 Cf. A213188.
%K A213189 nonn
%O A213189 1,1
%A A213189 _Antonio Roldán_, Feb 28 2013