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 A211201 #19 Jan 27 2022 13:28:01 %S A211201 0,1,2,6,5,10,19,22,37,77,108,165,90,313,461,620,1252,1957,2610,3237, %T A211201 5654,7797,9818,15797,22245,34725,56723,92634,122330,178540,226838, %U A211201 507571,454045,490426,1480005,2284378,1482842,2965594,5931098,10218573,11096982,21793257,31317157 %N A211201 Smallest m such that (sum of binary digits of m*(m+1)/2) = n. %H A211201 Donovan Johnson, <a href="/A211201/b211201.txt">Table of n, a(n) for n = 0..71</a> (terms < 2*10^12) %F A211201 A050493(a(n)) = n and A050493(m) <> n for m < a(n). %o A211201 (Haskell) %o A211201 import Data.List (elemIndex) %o A211201 import Data.Maybe (fromJust) %o A211201 a211201 = fromJust . (`elemIndex` a050493_list) %o A211201 (PARI) a(n) = my(m=0); while (hammingweight(m*(m+1)/2) != n, m++); m; \\ _Michel Marcus_, Jan 27 2022 %Y A211201 Cf. A089999, A050493, A000217, A000120. %K A211201 nonn,base %O A211201 0,3 %A A211201 _Reinhard Zumkeller_, Feb 04 2013