%I #10 Feb 19 2022 04:41:47

%S 0,3,6,10,15,18,21,28,30,36,45,55,60,63,66,78,84,90,91,105,108,120,

%T 126,135,136,150,153,165,168,171,190,198,210,216,231,234,253,270,273,

%U 276,280,300,315,325,330,351,360,378,396,406,408,420,435,450,459,465,468

%N Numbers of the form x*(x + 1)*y*(y + 1)/4 where x and y are distinct.

%F a(n) = A053990(n) / 4. - _Sean A. Irvine_, Feb 19 2022

%t 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 Cf. A053990, A054734. Contains all triangular numbers >1.

%K nonn,easy

%O 1,2

%A Stuart M. Ellerstein (ellerstein(AT)aol.com), Apr 22 2000

%E More terms from _James A. Sellers_, Apr 22 2000