A143731 - OEIS

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A143731

Characteristic function of numbers with at least two distinct prime factors (A024619).

11

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, 1, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1

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OFFSET

1,1

COMMENTS

a(n) = 1 iff n is divisible by at least two distinct primes; otherwise 0.

LINKS

Reikku Kulon, Table of n, a(n) for n = 1..10000

Index entries for characteristic functions

FORMULA

a(n) = abs(A079872(n)) = A057427(A079275(n)).

For n>1: a(n) = 1 - floor(1/A001221(n)). - Enrique Pérez Herrero, Aug 13 2012

MATHEMATICA

Table[Boole[PrimeNu[n]>1], {n, 1, 200}] (* Enrique Pérez Herrero, Aug 13 2012 *)

CROSSREFS

Cf. A024619, A079275, A079872, A000961, A001222, A091304, A073093, A001221.

Sequence in context: A353672 A323170 A215480 * A079872 A235046 A280046

Adjacent sequences: A143728 A143729 A143730 * A143732 A143733 A143734

KEYWORD

easy,nonn

AUTHOR

Reikku Kulon, Aug 30 2008

STATUS

approved