%I #17 Aug 19 2022 14:10:10
%S 0,0,0,0,0,1,0,0,0,1,0,1,0,1,1,0,0,1,0,1,1,1,0,1,0,1,0,1,0,0,0,0,1,1,
%T 1,1,0,1,1,1,0,0,0,1,1,1,0,1,0,1,1,1,0,1,1,1,1,1,0,0,0,1,1,0,1,0,0,1,
%U 1,0,0,1,0,1,1,1,1,0,0,1,0,1,0,0,1,1,1
%N Characteristic function of numbers n with exactly two distinct prime factors.
%C Characteristic function of numbers A007774.
%C Sum_{n>=1} a(n)/n^s = Sum_(A001221(n)=2, 1/n^s) = 1/2((Sum_(p prime, 1/(p^s-1))^2 - Sum_(p prime, 1/(p^s-1)^2)). - _Enrique Pérez Herrero_, Aug 14 2012
%H Enrique Pérez Herrero, <a href="/A215480/b215480.txt">Table of n, a(n) for n = 1..5000</a>
%F a(n) = (1-floor(1/A001221(n))*floor(2/A001221(n)), n > 1.
%t Table[Boole[PrimeNu[n]==2],{n,1,200}]
%Y Cf. A001221, A007774, A143731.
%K nonn
%O 1
%A _Enrique Pérez Herrero_, Aug 12 2012