%I #13 Mar 11 2023 12:44:03
%S 21,28,276,1540
%N Triangular numbers that are the sum of exactly three distinct powers of 2.
%C Triangular numbers with exactly three ones in their binary representation: A000120(a(n)) = 3; triangular numbers in A014311; no more terms less than A014311(1000000) = 6129982174881536975083663305496791903885182827960991744.
%t Select[Accumulate[Range[60]],Total[IntegerDigits[#,2]]==3&] (* _Harvey P. Dale_, Mar 11 2023 *)
%o (Haskell)
%o a212192 n = a212192_list !! (n-1)
%o a212192_list = filter ((== 1) . a010054) a014311_list
%Y Cf. A010054, A212190.
%K nonn
%O 1,1
%A _Reinhard Zumkeller_, May 03 2012