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 A212192 #13 Mar 11 2023 12:44:03 %S A212192 21,28,276,1540 %N A212192 Triangular numbers that are the sum of exactly three distinct powers of 2. %C A212192 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 A212192 Select[Accumulate[Range[60]],Total[IntegerDigits[#,2]]==3&] (* _Harvey P. Dale_, Mar 11 2023 *) %o A212192 (Haskell) %o A212192 a212192 n = a212192_list !! (n-1) %o A212192 a212192_list = filter ((== 1) . a010054) a014311_list %Y A212192 Cf. A010054, A212190. %K A212192 nonn %O A212192 1,1 %A A212192 _Reinhard Zumkeller_, May 03 2012