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 A215480 Characteristic function of numbers n with exactly two distinct prime factors 2
 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, 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, 1, 0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1 COMMENTS Characteristic function of numbers A007774. 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 LINKS Enrique Pérez Herrero, Table of n, a(n) for n = 1..5000 FORMULA for n>1: a(n)=(1-floor(1/A001221(n))*floor(2/A001221(n)). MATHEMATICA Table[Boole[PrimeNu[n]==2], {n, 1, 200}] CROSSREFS Cf. A143731 Sequence in context: A231600 A280710 A323170 * A143731 A079872 A235046 Adjacent sequences:  A215477 A215478 A215479 * A215481 A215482 A215483 KEYWORD nonn AUTHOR Enrique Pérez Herrero, Aug 12 2012 STATUS approved

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Last modified April 5 13:14 EDT 2020. Contains 333241 sequences. (Running on oeis4.)