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 A151774 #12 Mar 31 2021 04:34:25
%S A151774 0,0,0,1,0,1,1,0,0,1,1,0,1,0,0,0,0,1,1,0,1,0,0,0,1,0,0,0,0,0,0,0,0,1,
%T A151774 1,0,1,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,1,0,
%U A151774 1,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0
%N A151774 Characteristic function of numbers with binary weight 2 (A018900).
%C A151774 a(A018900(n)) = 1; a(A161989(n)) = 0. - _Reinhard Zumkeller_, Jun 24 2009
%H A151774 R. Zumkeller, <a href="/A151774/b151774.txt">Table of n, a(n) for n = 0..10000</a>
%H A151774 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%F A151774 Let Theta = Sum_{k >= 0} x^(2^k). G.f. is (x + Theta^2 - Theta)/2 (cf. A151758).
%t A151774 w[n_] := IntegerDigits[n, 2] // Total;
%t A151774 a[n_] := Boole[w[n] == 2];
%t A151774 a /@ Range[0, 105] (* _Jean-François Alcover_, Mar 31 2021 *)
%o A151774 (PARI) a(n)=hammingweight(n)==2 \\ _Charles R Greathouse IV_, Sep 26 2015
%Y A151774 Cf. A000120, A018900, A151758.
%K A151774 nonn,easy
%O A151774 0,1
%A A151774 _N. J. A. Sloane_, Jun 23 2009