A215480 Characteristic function of numbers n with exactly two distinct prime factors.

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

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

a(n) = (1-floor(1/A001221(n)))*floor(2/A001221(n)), n > 1.

Mathematica

Table[Boole[PrimeNu[n]==2], {n, 1, 200}]

Crossrefs

Cf. A001221, A007774, A143731.

Sequence in context: A353672 A323170 A379118 * A143731 A079872 A235046

Adjacent sequences: A215477 A215478 A215479 * A215481 A215482 A215483

Keyword

nonn,easy

Author

Enrique Pérez Herrero, Aug 12 2012

Status

approved