A143731 Characteristic function of numbers with at least two distinct prime factors (A024619).

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

Offset: 1

Author

Reikku Kulon, Aug 30 2008

Keywords

• easy
• nonn

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

Crossrefs

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

Cf. A215480.

Programs

Mathematica

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

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