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A054731
Numbers of the form x*(x + 1)*y*(y + 1)/4 where x and y are distinct.
5
0, 3, 6, 10, 15, 18, 21, 28, 30, 36, 45, 55, 60, 63, 66, 78, 84, 90, 91, 105, 108, 120, 126, 135, 136, 150, 153, 165, 168, 171, 190, 198, 210, 216, 231, 234, 253, 270, 273, 276, 280, 300, 315, 325, 330, 351, 360, 378, 396, 406, 408, 420, 435, 450, 459, 465, 468
OFFSET
1,2
FORMULA
a(n) = A053990(n) / 4. - Sean A. Irvine, Feb 19 2022
MATHEMATICA
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 *)
CROSSREFS
Cf. A053990, A054734. Contains all triangular numbers >1.
Sequence in context: A194458 A083266 A334130 * A334129 A282064 A029716
KEYWORD
nonn,easy
AUTHOR
Stuart M. Ellerstein (ellerstein(AT)aol.com), Apr 22 2000
EXTENSIONS
More terms from James A. Sellers, Apr 22 2000
STATUS
approved