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 A054731 #11 Jul 02 2025 16:01:59
%S A054731 0,3,6,10,15,18,21,28,30,36,45,55,60,63,66,78,84,90,91,105,108,120,
%T A054731 126,135,136,150,153,165,168,171,190,198,210,216,231,234,253,270,273,
%U A054731 276,280,300,315,325,330,351,360,378,396,406,408,420,435,450,459,465,468
%N A054731 Numbers of the form x*(x + 1)*y*(y + 1)/4 where x and y are distinct.
%F A054731 a(n) = A053990(n) / 4. - _Sean A. Irvine_, Feb 19 2022
%t A054731 With[{upto=500},Select[Union[(#[[1]](#[[1]]+1)#[[2]](#[[2]]+1))/4&/@ Subsets[ Range[0,Floor[upto/2]],{2}]],#<=upto&]] (* _Harvey P. Dale_, Jan 15 2015 *)
%Y A054731 Cf. A053990, A054734. Contains all triangular numbers >1.
%K A054731 nonn,easy
%O A054731 1,2
%A A054731 Stuart M. Ellerstein (ellerstein(AT)aol.com), Apr 22 2000
%E A054731 More terms from _James Sellers_, Apr 22 2000