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